rtl8139: Fix compilation for w32/w64

Compilation for Windows needs a different declaration for the
printf format attribute, so use the macro which was defined for
this purpose.

Cc: Benjamin Poirier <benjamin.poirier@gmail.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

hw/rtl8139.c

@@ -88,8 +88,7 @@
 #  define DPRINTF(fmt, ...) \
     do { fprintf(stderr, "RTL8139: " fmt, ## __VA_ARGS__); } while (0)
 #else
-static inline __attribute__ ((format (printf, 1, 2)))
-    int DPRINTF(const char *fmt, ...)
+static inline GCC_FMT_ATTR(1, 2) int DPRINTF(const char *fmt, ...)
 {
     return 0;
 }