xorg: depend on WCHAR and LOCALE

Xorg needs WCHAR and LOCALE, so make sure it cannot be selected unless
this is enabled in the toolchain.

We unfortunately cannot make BR2_XSERVER_xorg depend on BR2_USE_LOCALE
because of a strange recursion error with dbus, but now we atleast
inform the user what to do to get a working configuration.

The real solution is ofcause of fix xorg, but that's for another day.