gluon-radv-filterd: add string.h and guard to mac.h

- #pragma once include guard
- include string.h to get memset and memcmp prototypes to fix respondd
  provider build warning

(cherry picked from commit e32fa5bc)

package/gluon-radv-filterd/src/mac.h

+#pragma once
+
 #include <stdint.h>
+#include <string.h>
 #include <linux/if_ether.h>
 
 #define F_MAC "%02hhx:%02hhx:%02hhx:%02hhx:%02hhx:%02hhx"