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>