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

Qt 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.