netfilter: xt_LOG: add __printf() to sb_add()

Helps to find format mismatches at compile time
Suggested-by: NDavid Laight <David.Laight@ACULAB.COM>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>

include/net/netfilter/xt_log.h

@@ -6,7 +6,7 @@ struct sbuff {
 };
 static struct sbuff emergency, *emergency_ptr = &emergency;
 
-static int sb_add(struct sbuff *m, const char *f, ...)
+static __printf(2, 3) int sb_add(struct sbuff *m, const char *f, ...)
 {
 	va_list args;
 	int len;