gitignore: only ignore *local*.conf" under mkosi.default.d/

The pattern was added in 6242cda9, with the
idea that users will have local configuration files for mkosi and git should
not bother them about those. But let's make this narrower, and only match
files with "local". This way we reduce the risk that some unrelated file
will be ignored by accident.

.gitignore in the parent directory is used, because mkosi apparently tries
to load all files under mkosi.default.d/, without looking at the extension.
This is probably something to fix in mkosi too.