w32: Add another fallback to look for gpgconf.exe

* src/w32-util.c (_gpgme_get_gpgconf_path): Add fallback for
GnuPG VS-Desktop install scheme.

GnuPG VS-Desktop has the install layout:

%PROGRAMFILES%\GnuPG VS-Desktop\bin\libgpgme.dll
%PROGRAMFILES%\GnuPG VS-Desktop\GnuPG\bin\gpgconf.exe

This fallback is of course only required if the registry
string pointing to the correct install directory is
broken.

• GnuPG-Bug-Id: T6131