zenfone9: Set TARGET_PREBUILT_KERNEL

Change-Id: I38ffe7ef9a7a299316d9f240f3db7722eaf3ba1f
diff --git a/device.mk b/device.mk
index 612c271..6a39479 100755
--- a/device.mk
+++ b/device.mk
@@ -30,6 +30,7 @@
 # Prebuilt Kernel Binary
 TARGET_KERNEL_VERSION := 5.10
 TARGET_KERNEL_DIR ?= device/asus/zenfone9-kernel
+TARGET_PREBUILT_KERNEL := device/asus/zenfone9-kernel/Image
 LOCAL_KERNEL := $(TARGET_KERNEL_DIR)/Image
 PRODUCT_COPY_FILES += \
     $(LOCAL_KERNEL):kernel