qt: remove references to /usr/{share,lib}/X11/

Qt currently hardcodes /usr/{share,lib}/X11/locale/ and
/usr/share/X11/xkb/. Fix it to use paths from the nix store.

This fixes qtcreator startup warning (and maybe other issues):

  Qt Warning: Could not find a location of the system's Compose files. Consider setting the QTCOMPOSE environment variable.