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.
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