net: add missing includes and forward declarations under net/

This patch adds missing includes to headers under include/net.
All these problems are currently masked by the existing users
including the missing dependency before the broken header.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/llc_s_ac.h b/include/net/llc_s_ac.h
index a61b98c..f717903 100644
--- a/include/net/llc_s_ac.h
+++ b/include/net/llc_s_ac.h
@@ -11,6 +11,10 @@
  *
  * See the GNU General Public License for more details.
  */
+
+struct llc_sap;
+struct sk_buff;
+
 /* SAP component actions */
 #define SAP_ACT_UNITDATA_IND	1
 #define SAP_ACT_SEND_UI		2