hikey960.mk: set HIKEY_USES_GKI true for mainline kernel

to avoid invonvenience when forgot to set explicitly,
which will cause boot failures on the adb connections

Test: booted to homescreen with the mainline kernel
      and adb connection works

Change-Id: I07e22d831a747915e7926fba584b0665c1d3f558
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
diff --git a/hikey960.mk b/hikey960.mk
index 3a58d25..673181c 100644
--- a/hikey960.mk
+++ b/hikey960.mk
@@ -8,6 +8,10 @@
 ifndef HIKEY_USES_GKI
   ifeq ($(TARGET_KERNEL_USE), 5.4)
     HIKEY_USES_GKI := true
+  else
+    ifeq ($(TARGET_KERNEL_USE), mainline)
+      HIKEY_USES_GKI := true
+    endif
   endif
 endif