selinux: sparse fix: include selinux.h in exports.c

Fix warning:
security/selinux/exports.c:18:6: warning: symbol 'selinux_is_enabled' was not declared. Should it be static?
Signed-off-by: James Morris <jmorris@namei.org>

security/selinux/exports.c

@@ -12,6 +12,7 @@
  * as published by the Free Software Foundation.
  */
 #include <linux/module.h>
+#include <linux/selinux.h>
 
 #include "security.h"