Replace GCC_FMT_ATTR with G_GNUC_PRINTF

One less qemu-specific macro. It also helps to make some headers/units
only depend on glib, and thus moved in standalone projects eventually.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Richard W.M. Jones <rjones@redhat.com>

qemu-io.c

@@ -329,7 +329,7 @@ static char *get_prompt(void)
     return prompt;
 }
 
-static void GCC_FMT_ATTR(2, 3) readline_printf_func(void *opaque,
+static void G_GNUC_PRINTF(2, 3) readline_printf_func(void *opaque,
                                                     const char *fmt, ...)
 {
     va_list ap;