.gitignore: Add an exception for Kconfig.debug

Change-Id: I70f19497a2cb1314a1ab18a3da5fd47e40fb57cb
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36171
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

.gitignore

@@ -56,6 +56,7 @@ site-local
 *.\#
 *.bin
 *.debug
+!Kconfig.debug
 *.elf
 *.o
 *.out