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

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

Change-Id: I4e3b1915e6ce89356f6d97f8fa2076a6f2174395
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>