mb/google/rex: Add SPDX license headers to template Makefile

According to CB:68982, add SPDX license headers to rex template
Makefile. Without this patch, build bot may report build failed.

BUG=none
TEST=Run create_coreboot_variant.sh and check Makefile

Change-Id: I7d100a2303be063590f1389a7eed8f9a52071241
Signed-off-by: Tyler Wang <tyler.wang@quanta.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/84712
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <ericllai@google.com>
Reviewed-by: Subrata Banik <subratabanik@google.com>

util/mainboard/google/rex0/template/Makefile.mk

@@ -1,3 +1,5 @@
+## SPDX-License-Identifier: GPL-2.0-only
+
 bootblock-y += gpio.c
 
 romstage-y += gpio.c