Avoid gnu_printf attribute when using Clang

Clang does not support gnu_printf, so just use printf when using it to
compile.

Signed-off-by: Ed Maste <emaste@freebsd.org>
Message-Id: <20191120210422.61327-1-emaste@freefall.freebsd.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

util.h

@@ -12,7 +12,11 @@
  */
 
 #ifdef __GNUC__
+#ifdef __clang__
+#define PRINTF(i, j)	__attribute__((format (printf, i, j)))
+#else
 #define PRINTF(i, j)	__attribute__((format (gnu_printf, i, j)))
+#endif
 #define NORETURN	__attribute__((noreturn))
 #else
 #define PRINTF(i, j)