.gitignore: Do not ignore .kunitconfig files

Circumvent the .gitignore wildcard to avoid warnings about ignored
.kunitconfig files. As far as I can tell, the warnings are harmless
and these files are not actually ignored.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202304142337.jc4oUrov-lkp@intel.com/
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>