installer: Add mingw32-libexpat as a requirement for building git

Signed-off-by: Thomas Braun <thomas.braun@virtuell-zuhause.de>
Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Add an mgwport file for mingw32-git

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>