net: fix build error if CONFIG_CGROUPS=n

Reported-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -64,6 +64,8 @@
 #include <net/dst.h>
 #include <net/checksum.h>
 
+struct cgroup;
+struct cgroup_subsys;
 int mem_cgroup_sockets_init(struct cgroup *cgrp, struct cgroup_subsys *ss);
 void mem_cgroup_sockets_destroy(struct cgroup *cgrp, struct cgroup_subsys *ss);
 /*