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/include/hw/hw.h b/include/hw/hw.h
index fc5301f..34377f5 100644
--- a/include/hw/hw.h
+++ b/include/hw/hw.h
@@ -5,6 +5,6 @@
 #error Cannot include hw/hw.h from user emulation
 #endif
 
-void QEMU_NORETURN hw_error(const char *fmt, ...) GCC_FMT_ATTR(1, 2);
+void QEMU_NORETURN hw_error(const char *fmt, ...) G_GNUC_PRINTF(1, 2);
 
 #endif