xkbvalidate: Rename output binary to xkbvalidate

So far, the output binary has been just "validate", which is quite a
very generic name and doesn't match the package name.

Even though I highly doubt that this program will ever be used outside
of NixOS modules, it's nevertheless less confusing to have a consistent
naming.

Signed-off-by: aszlig <aszlig@nix.build>

nixos/modules/services/x11/xserver.nix

@@ -714,7 +714,7 @@ in
       nativeBuildInputs = [ pkgs.xkbvalidate ];
       preferLocalBuild = true;
     } ''
-      validate "$xkbModel" "$layout" "$xkbVariant" "$xkbOptions"
+      xkbvalidate "$xkbModel" "$layout" "$xkbVariant" "$xkbOptions"
       touch "$out"
     '');