zmq2edi: use int for getopt return value

Because somebody reported
> src/zmq2edi/zmq2edi.cpp:231:19: warning: case label value is less than
> minimum value for type
for the case -1:

1 files changed, 1 insertions, 1 deletions

diff --git a/src/zmq2edi/zmq2edi.cpp b/src/zmq2edi/zmq2edi.cpp
index ebebc17..b2c20f4 100644
--- a/src/zmq2edi/zmq2edi.cpp
+++ b/src/zmq2edi/zmq2edi.cpp
@@ -224,7 +224,7 @@ int start(int argc, char **argv)
 
     int delay_ms = 500;
 
-    char ch = 0;
+    int ch = 0;
     while (ch != -1) {
         ch = getopt(argc, argv, "d:p:s:S:t:Pf:i:Dva:w:");
         switch (ch) {