.gitignore: Unignore .kunitconfig

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

Signed-off-by: Andy Shevchenko <[e-mail]>
Reviewed-by: David Gow <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

.gitignore

···
 !.get_maintainer.ignore
 !.gitattributes
 !.gitignore
+!.kunitconfig
 !.mailmap
 !.rustfmt.toml