License: make clear on ktap license

ktap is licensed ad GPL, but since it's based on lua initially,
so here I give credit to lua.

lua relased as MIT, but that's not a problem, because MIT license
is compatible with GPL, so it's not violation with ktap's GPL license
(checked this with lua team)

See more info in doc/ktap_license.txt

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@huawei.com>