.mailmap: add entry for Zhiguang Liu

... for git-shortlog purposes.

Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Zhiguang Liu <zhiguang.liu@intel.com>