libxkbcommon: Enable libxkbregistry

This also enables the "list" subcommand of xkbcli.

Since libxkbregistry is an optional library we could install it into a
different output. However, doing this properly is quite challenging and
the best approach would likely be to upstream patches that add a Meson
option for installing libxkbregistry under a separate prefix (so that
the pkg-config file is generated correctly, etc.).
But even then the default fixup phase would try to move
$libxkbregistry/include into the $dev output and the $out output would
depend on the $libxkbcommon output because of the xkbcli binary (though
we could move that into a $bin output).
As a result it seems best not to install libxkbregistry into a dedicated
output path.

pkgs/development/libraries/libxkbcommon/default.nix

@@ -31,7 +31,6 @@ stdenv.mkDerivation rec {
     "-Dxkb-config-root=${xkeyboard_config}/etc/X11/xkb"
     "-Dxkb-config-extra-path=/etc/xkb" # default=$sysconfdir/xkb ($out/etc)
     "-Dx-locale-root=${libX11.out}/share/X11/locale"
-    "-Denable-xkbregistry=false" # Optional, separate library (TODO: Install into extra output)
     "-Denable-wayland=${lib.boolToString withWaylandSupport}"
   ];