mailmap: Add WeiXiong Liao

WeiXiong Liao noted to me offlist that his preference for email address
had changed and that he'd like it updated in the mailmap so people
discussing pstore/blk would be able to reach him.

Cc: WeiXiong Liao <gmpy.liaowx@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>