w32: Support utf8 for getcwd even if build with gpgrt < 1.40.

* common/sysutils.c (gnupg_getcwd) [W32]: Use Unicode version.

gpgrt 1.40 has not yet been released, so to make getcwd work properly
on Windows we need to have the code here as well.

• GnuPG-bug-id: T5098
• Signed-off-by: Werner Koch <wk@gnupg.org>