Compile fix for "peer secid consolidation for external network labeling"

Use a forward declaration instead of dragging in skbuff.h and
related junk.
Signed-off-by: James Morris <jmorris@namei.org>

security/selinux/include/security.h

@@ -8,7 +8,6 @@
 #ifndef _SELINUX_SECURITY_H_
 #define _SELINUX_SECURITY_H_
 
-#include <linux/skbuff.h>
 #include "flask.h"
 
 #define SECSID_NULL			0x00000000 /* unspecified SID */
@@ -35,6 +34,8 @@
 #define POLICYDB_VERSION_MAX	POLICYDB_VERSION_RANGETRANS
 #endif
 
+struct sk_buff;
+
 extern int selinux_enabled;
 extern int selinux_mls_enabled;