util/lint: Add SPDX checker for makefiles

Now that all of the makefiles under the src tree have SPDX headers,
add that expectation to the license header lint tool by removing
the exception for Makefile.inc.

Signed-off-by: Martin Roth <gaumless@gmail.com>
Change-Id: Iab9d3262621af09a1c625378ae2e61e8a736cdf8
Reviewed-on: https://review.coreboot.org/c/coreboot/+/76952
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

util/lint/lint-000-license-headers

@@ -61,7 +61,6 @@ Dockerfile|\
 _shipped$|\
 /microcode-[^/]*.h$|\
 /sdram-.*\.inc$|\
-Makefile\.inc|\
 \.fmd|\
 \.cb|\
 \.cfg$|\