hikey960.mk: set HIKEY_USES_GKI true for mainline kernel am: ca30a6e803 am: c1c3214384 am: 0780948d8f

Original change: https://android-review.googlesource.com/c/device/linaro/hikey/+/2373271

Change-Id: Ie8d93f105d8d1dad61478f655908af506b8bbfc8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>