Put #ifdef/#endif around a variable used only on Windows.

Change-Id: Ia6fed6db03cb88433469e758693fcc72c4bd9071
Reviewed-on: https://code.wireshark.org/review/16009
Reviewed-by: Guy Harris <guy@alum.mit.edu>

wireshark-qt.cpp

@@ -313,7 +313,10 @@ int main(int argc, char *argv[])
 {
     MainWindow *main_w;
 
-    int                  opt, ret_val;
+#ifdef _WIN32
+    int                  opt;
+#endif
+    int                  ret_val;
     char               **ws_argv = argv;
 
 #ifdef _WIN32