Move utility function from win32.c to win32-util.c

Move utility function from win32.c to win32-util.c

This done to allow to include parts win32.c when building unit tests
as win32.c itself has too many dependencies and cannot be included in
a small unit test.

Also fix a missing Windows.h include in error.h that otherwise
breaks complation when included from unit tests.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210512131511.1309914-8-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22348.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>