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

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

Change-Id: Ife4cb6c5775da370eb4122d6f7bae308a56f3417
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>