Move libdbus-init-win.cpp to its own convenience library

Move libdbus-init-win.cpp to its own convenience library

Otherwise libdbus-1 ends up linked as if it contained C++, even
on Unix, where it doesn't; in turn, that makes it export all the
underscore-prefixed symbols that aren't meant to be exported.

Reviewed-by: Thiago Macieira <thiago@kde.org>
Reviewed-by: David Zeuthen <davidz@redhat.com>