kconfig: remove lkc_defs.h from .gitignore and dontdiff

Commit 5a6f8d2b ("kconfig: nuke
LKC_DIRECT_LINK cruft") removed all traces of lkc_defs.h from the tree.
Remove its entries in dontdiff and kconfig's .gitignore file too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Michal Marek <mmarek@suse.cz>