w32: Use UTF-8 for localized error descriptions for all threads

* src/version.c (do_subsystem_inits) [W32]: Switch gettext to UTF-8
for current thread and all new threads.

We did already enable UTF-8, but it was only effective for the main
thread. Now we enable it also for all new threads (if supported by
libgpg-error). This way debug output in background threads now also
uses UTF-8.

• GnuPG-bug-id: T7188