maintainers: add two folks
maintainers: add two folks

The image-tools has received heavy focus and contribution from Xie
Keyang and Lie Jitang from Huawei.

I propose add them so we can keep the progress rolling on the tools
repo!

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>

xiekeyang/image-tools,opencontainers/image-tools,opencontainers/image-tools,xiekeyang/image-tools