Add libXt documentation directory. spotted by naddy@

src/etc/mtree/BSD.x11.dist

@@ -1,4 +1,4 @@
-#	$OpenBSD: BSD.x11.dist,v 1.23 2012/03/10 14:02:52 matthieu Exp $
+#	$OpenBSD: BSD.x11.dist,v 1.24 2012/03/10 21:33:35 matthieu Exp $
 
 /set type=dir uname=root gname=wheel mode=0755
 .
@@ -368,6 +368,8 @@
 		    ..
 		    libXrender
 		    ..
+		    libXt
+		    ..
 		    libXtst
 		    ..
 		    libXvMC