utils: Fix string format message

Fix the format string to match the size_t type, this fixes a build
problem on 64 bit platforms.

Fixes: 4d0c703e750c ("firewall3: Fix some format string problems")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

utils.c

@@ -145,7 +145,7 @@ fw3_alloc(size_t size)
 	mem = calloc(1, size);
 
 	if (!mem)
-		error("Out of memory while allocating %d bytes", size);
+		error("Out of memory while allocating %zd bytes", size);
 
 	return mem;
 }