Suppress warning of unused function on macOS

This function is only used on FreeBSD and Linux.

forkfd.c:243:12: warning: unused function 'convertForkfdWaitFlagsToWaitFlags' [-Wunused-function]

Pick-To: 5.15
Change-Id: I99ab0f318b1c43b89888fffd160bf81f01960f2f
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/3rdparty/forkfd/forkfd.c

@@ -240,6 +240,9 @@ static void convertStatusToForkfdInfo(int status, struct forkfd_info *info)
     }
 }
 
+#ifdef __GNUC__
+__attribute__((unused))
+#endif
 static int convertForkfdWaitFlagsToWaitFlags(int ffdoptions)
 {
     int woptions = WEXITED;