enum: Fix compiler warning

Closes strongswan/strongswan#1025

src/libstrongswan/utils/enum.c

@@ -97,7 +97,7 @@ char *enum_flags_to_string(enum_name_t *e, u_int val, char *buf, size_t len)
 		return buf;
 	}
 
-	if (snprintf(buf, len, e->names[0]) >= len)
+	if (snprintf(buf, len, "%s", e->names[0]) >= len)
 	{
 		return NULL;
 	}