add missing bpf-cgroup.h includes

We're about to break the cgroup-defs.h -> bpf-cgroup.h dependency,
make sure those who actually need more than the definition of
struct cgroup_bpf include bpf-cgroup.h explicitly.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/bpf/20211216025538.1649516-3-kuba@kernel.org

net/socket.c

@@ -52,6 +52,7 @@
  *	Based upon Swansea University Computer Society NET3.039
  */
 
+#include <linux/bpf-cgroup.h>
 #include <linux/ethtool.h>
 #include <linux/mm.h>
 #include <linux/socket.h>