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 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.

----
There are very few packages using qt5 in nixpkgs, so maybe we can merge this directly to master?