Fixing the libxcb native build: missing dependency.

svn path=/nixpkgs/branches/stdenv-updates/; revision=18499

pkgs/servers/x11/xorg/overrides.nix

@@ -21,6 +21,10 @@
     preBuild = "substituteInPlace mkfontdir.cpp --replace BINDIR ${xorg.mkfontscale}/bin";
   };
 
+  libxcb = attrs : attrs // {
+    buildInputs = attrs.buildInputs ++ [ xorg.xproto ];
+  };
+
   libXext = attrs: attrs // {
     buildInputs = attrs.buildInputs ++ [xorg.libXau];
   };