xmcp: move to X11 directory

The lowercase x11 directory was causing errors
on macOS machines.

pkgs/top-level/all-packages.nix

···
 
   xsos = callPackage ../os-specific/linux/xsos { };
 
-  xmcp = callPackage ../tools/x11/xmcp { };
+  xmcp = callPackage ../tools/X11/xmcp { };
 
   zk = callPackage ../applications/office/zk {};