host: x4xx: Fix some warnings on mac OS

This fixes the following warnings:
```
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:2335:5: warning: \
delete called on non-final 'uhd::rfnoc::x400::x400_gpio_port_mapping' that has \
virtual functions but non-virtual destructor [-Wdelete-non-virtual-dtor]
    delete __ptr;
    ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:2648:7: note: in \
instantiation of member function 'std::__1::default_delete<uhd::rfnoc::x400::x4\
00_gpio_port_mapping>::operator()' requested here
      __ptr_.second()(__tmp);
      ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:2602:19: note: in\
instantiation of member function 'std::__1::unique_ptr<uhd::rfnoc::x400::x400_g\
pio_port_mapping, std::__1::default_delete<uhd::rfnoc::x400::x400_gpio_port_map\
ping> >::reset' requested here
  ~unique_ptr() { reset(); }
                  ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:4063:21: note: in\
instantiation of member function 'std::__1::unique_ptr<uhd::rfnoc::x400::x400_g\
pio_port_mapping, std::__1::default_delete<uhd::rfnoc::x400::x400_gpio_port_map\
ping> >::~unique_ptr' requested here
    unique_ptr<_Yp> __hold(__p);
                    ^
/Users/rfmibuild/myagent/_work/76/s/host/lib/usrp/x400/x400_radio_control.cpp:1\
92:33: note: in instantiation of function template specialization 'std::__1::sh\
ared_ptr<uhd::mapper::gpio_port_mapper>::shared_ptr<uhd::rfnoc::x400::x400_gpio\
_port_mapping>' requested here
        auto gpio_port_mapper = std::shared_ptr<uhd::mapper::gpio_port_mapper>(
```
and:
```
/Users/rfmibuild/myagent/_work/76/s/host/lib/usrp/x400/x400_gpio_control.cpp:15\
4:75: warning: adding 'const uint32_t' (aka 'const unsigned int') to a string d\
oes not append to the string [-Wstring-plus-int]
        "Could not find corresponding GPIO pin number for given SPI pin " + value);
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~
/Users/rfmibuild/myagent/_work/76/s/host/lib/usrp/x400/x400_gpio_control.cpp:15\
4:75: note: use array indexing to silence this warning
        "Could not find corresponding GPIO pin number for given SPI pin " + value);
                                                                          ^
        &                                                                 [      ]
```

2 files changed, 3 insertions, 2 deletions

diff --git a/host/lib/usrp/x400/x400_gpio_control.cpp b/host/lib/usrp/x400/x400_gpio_control.cpp
index 706364e87..b0eb4f27e 100644
--- a/host/lib/usrp/x400/x400_gpio_control.cpp
+++ b/host/lib/usrp/x400/x400_gpio_control.cpp
@@ -151,8 +151,7 @@ uint32_t uhd::rfnoc::x400::x400_gpio_port_mapping::map_value(const uint32_t& val
         }
     }
     throw uhd::lookup_error(
-        "Could not find corresponding GPIO pin number for given SPI pin " + value);
-    return 0;
+        std::string("Could not find corresponding GPIO pin number for given SPI pin ") + std::to_string(value));
 }
 
 uint32_t uhd::rfnoc::x400::x400_gpio_port_mapping::unmap_value(const uint32_t& value)
diff --git a/host/lib/usrp/x400/x400_gpio_control.hpp b/host/lib/usrp/x400/x400_gpio_control.hpp
index 4e5aed587..492413685 100644
--- a/host/lib/usrp/x400/x400_gpio_control.hpp
+++ b/host/lib/usrp/x400/x400_gpio_control.hpp
@@ -21,6 +21,8 @@ class x400_gpio_port_mapping : public uhd::mapper::gpio_port_mapper
 public:
     x400_gpio_port_mapping(){};
 
+    virtual ~x400_gpio_port_mapping() = default;
+
     uint32_t map_value(const uint32_t& value) override;
 
     uint32_t unmap_value(const uint32_t& value) override;