cgroup: net_cls: Move sock_update_classid() declaration to cls_cgroup.h

The only user of sock_update_classid() is net/socket.c which happens
to include cls_cgroup.h directly.

tj: Fix build breakage due to missing cls_cgroup.h inclusion in
    drivers/net/tun.c reported in linux-next by Stephen.

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Gao feng <gaofeng@cn.fujitsu.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: John Fastabend <john.r.fastabend@intel.com>
Cc: netdev@vger.kernel.org
Cc: cgroups@vger.kernel.org
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 3a16d4f..9336b82 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -68,6 +68,7 @@
 #include <net/netns/generic.h>
 #include <net/rtnetlink.h>
 #include <net/sock.h>
+#include <net/cls_cgroup.h>
 
 #include <asm/uaccess.h>