Adding gsk_keys.mk

Adds the following line into a device.mk in order to include
GSI keys into the first-stage ramdisk.

   $(call inherit-product, $(SRC_TARGET_DIR)/product/gsi_keys.mk)

Bug: 112293933
Test: make checks $OUT/ramdisk/avb/.
Change-Id: I8e4a9a8dd607916a3f45a07e0ba14c58def5d8d3