Add __FBSDID() tag.

MFC after:	3 days
Pointed out by:	antoine

sys/kern/kern_priv.c

@@ -25,12 +25,13 @@
  * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- *
- * $FreeBSD$
  */
 
 #include "opt_mac.h"
 
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
 #include <sys/param.h>
 #include <sys/jail.h>
 #include <sys/kernel.h>