xkb: Ensure strings from xkeyboard-config are UTF-8

When using g_dgettext, translated strings are returned in the
current locale's encoding. This means XKEYBOARD_CONFIG() can
give us a non-UTF-8 string if a non-UTF-8 locale is in use.
Calling bind_textdomain_codeset() ensures that the strings returned
by g_dgettext will be encoded in UTF-8, which is what applications
using gnome-desktop expect.

https://bugzilla.gnome.org/show_bug.cgi?id=703247