git-ignore site-local

It's _local_

Change-Id: I5624e240ffe486763b25b14b218e69362c488f50
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/5432
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>