Add a .clang-format file

How to use:

Install 'git-format-clang' which is part of the clang suite (Fedora:
git-clang-format, openSUSE: clang-tools).

Now do your changes and stage them with `git add`. Once they are staged
format the code using `git clang-format` before you commit.

Now the formatting changed can be viewed with `git diff` against the
staged changes.

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>

Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Wed Jul 20 18:23:49 UTC 2022 on sn-devel-184
