.gitignore: Ignore emacs backup file

This trivial change was motivated to confirm that gitlab can allow external community contributions.

For the record my affiliation has been changed from Huawei OSTC to Individual.

Forward: https://gitlab.eclipse.org/eclipse/oniro-core/oniro/-/merge_requests?author_username=pcoval&scope=all&state=all Signed-off-by: Philippe Coval philippe.coval@astrolabe.coop