include/gatekeeper_net.h: properly delay definition of struct gatekee…

…per_if

The header include/gatekeeper_net.h was delaying the definition of
struct gatekeeper_if with the line:

struct gatekeeper_if *iface;

Instead of having the line:

struct gatekeeper_if;

The newer GCC available on Ubuntu 22.04 caught the issue.

include/gatekeeper_net.h

@@ -102,7 +102,7 @@ enum {
  * Need forward declaration because acl_cb_func and struct gatekeeper_if
  * are circularly defined.
  */
-struct gatekeeper_if *iface;
+struct gatekeeper_if;
 typedef int (*acl_cb_func)(struct rte_mbuf **pkts, unsigned int num_pkts,
 	struct gatekeeper_if *iface);
 /* Format of function called when no rule matches in the IPv6 ACL. */