util/lint: Add a license check exception for .gitkeep files

A .gitkeep file is an unofficial convention used in Git to keep and
track empty directories, as Git does not track empty folders by default.

This could be needed when one mainboard variant has an include directory
but another doesn't. If the directory is added to the include, it could
be easier to just create an empty include directory with a .gitkeep file
in it to keep things from failing.

Change-Id: I34b2ffa4d748d82e26867ecd5b9149301300e6a1
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/89640
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Nicholas Sudsgaard <devel+coreboot@nsudsgaard.com>

util/lint/lint-000-license-headers

@@ -69,7 +69,8 @@ _shipped$|\
 config|\
 cmos\.layout|\
 cmos\.default|\
-\.apcb$\
+\.apcb$|\
+\.gitkeep$\
 "
 
 #space separated list of directories to test