qt5: remove references to /usr/{share,lib}/X11/locale/

Qt5 currently hardcodes /usr/{share,lib}/X11/locale/. Fix it to use
paths from the nix store. Qt4 is unaffected.

This fixes a startup warning for applications built with qt5 (e.g.
qtcreator, shotcut):

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