add basedir of the kernel link-kit

requested by ajacoutot@
OK tb@

src/etc/mtree/4.4BSD.dist

@@ -1,4 +1,4 @@
-#	$OpenBSD: 4.4BSD.dist,v 1.298 2017/04/26 14:53:11 sthen Exp $
+#	$OpenBSD: 4.4BSD.dist,v 1.299 2017/08/21 20:52:06 rpe Exp $
 
 /set type=dir uname=root gname=wheel mode=0755
 
@@ -414,6 +414,10 @@ usr
         calendar
         ..
 
+        # ./usr/share/compile
+        compile
+        ..
+
         # ./usr/share/dict
         dict
             papers