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>
diff --git a/qga/guest-agent-core.h b/qga/guest-agent-core.h
index 9d01ea9..29cd504 100644
--- a/qga/guest-agent-core.h
+++ b/qga/guest-agent-core.h
@@ -36,7 +36,7 @@
 bool ga_logging_enabled(GAState *s);
 void ga_disable_logging(GAState *s);
 void ga_enable_logging(GAState *s);
-void GCC_FMT_ATTR(1, 2) slog(const gchar *fmt, ...);
+void G_GNUC_PRINTF(1, 2) slog(const gchar *fmt, ...);
 void ga_set_response_delimited(GAState *s);
 bool ga_is_frozen(GAState *s);
 void ga_set_frozen(GAState *s);