.gitignore: Unignore .kunitconfig

There are almost dozen of .kunitconfig files that are ignored but
tracked. Unignore them.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>