Bug 1324617 added /other-licenses/nsis/Contrib/CityHash/cityhash/city.cpp
to the sources for browser/components/shell/, which gets linked into libxul
via browsercomps.
Bug 1348609 then made toolkit/xre/nsXREDirProvider.cpp use the same file,
which made toolkit depend on browser. That busts other XUL apps, which then
have to individually satisfy that dependency.
This change links city.cpp into libxul directly by compiling it as part of
toolkit/xre/.