gatchat: Use pragma to mask GFunc casting warning

gatchat/gatmux.c: In function ‘watch_dispatch’:
gatchat/gatmux.c:454:17: error: cast between incompatible function types from ‘GSourceFunc’ {aka ‘int (*)(void *)’} to ‘gboolean (*)(GIOChannel *, GIOCondition,  void *)’ {aka ‘int (*)(struct _GIOChannel *, enum <anonymous>,  void *)’} [-Werror=cast-function-type]
  GIOFunc func = (GIOFunc) callback;
                 ^

gatchat/gatmux.c

@@ -30,6 +30,8 @@
 #include <string.h>
 #include <alloca.h>
 
+#pragma GCC diagnostic ignored "-Wcast-function-type"
+
 #include <glib.h>
 
 #include "ringbuffer.h"