selinux: export initial SID contexts via selinuxfs

Make the initial SID contexts accessible to userspace via selinuxfs.
An initial use of this support will be to make the unlabeled context
available to libselinux for use for invalidated userspace SIDs.

Signed-off-by: James Carter <jwcart2@tycho.nsa.gov>
Acked-by:  Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
index bfe562c..b94378a 100644
--- a/security/selinux/include/security.h
+++ b/security/selinux/include/security.h
@@ -123,5 +123,7 @@
 }
 #endif /* CONFIG_NETLABEL */
 
+const char *security_get_initial_sid_context(u32 sid);
+
 #endif /* _SELINUX_SECURITY_H_ */