add /usr/X11R6/share/libdrm for recent libdrm versions

src/etc/mtree/BSD.x11.dist

@@ -1,4 +1,4 @@
-#       $OpenBSD: BSD.x11.dist,v 1.44 2017/02/26 16:51:18 matthieu Exp $
+#       $OpenBSD: BSD.x11.dist,v 1.45 2017/08/05 14:13:39 jsg Exp $
 
 /set type=dir uname=root gname=wheel mode=0755
 .
@@ -419,6 +419,8 @@
                     xtrans
                     ..
                 ..
+                libdrm
+                ..
                 mk
                 ..
 		util-macros