Enforce C/C++ code style across postmarketOS repositories

We had enforced C code linting across all .c files in pmaports.git (pmaports!465). Later we decided to split the sources into their own repositories (which makes sense), but now we don't enforce a consistent code style. This is also the case for osk-sdl for example, for which the source never was part of pmaports.git.

The goal is to make it easy to integrate the check in all project's .gitlab-ci.yml:

clang-format:
  before_script:
    - apk -q add git
    - git clone -q --depth=1 "https://gitlab.postmarketos.org/postmarketos/ci-common.git"
  script:
    - ci-common/clang-format/ci_run.sh

To implement this, I suggest to fork ci-common.git and osk-sdl.git. Add it to osk-sdl.git's .gitlab-ci.yml, but change the clone path to point to your fork.

In ci-common.git, let's add a new subdirectory:

clang-format/
  clang-format.sh
  .clang-format
  ci_run.sh

clang-format.sh and .clang-format should be based on pmaports!465. Extend clang-format.sh to find the clang-format file from the subdir, and to work with c++ files too.

ci_run.sh should do apk -q add clang (so we can easily adjust dependencies if necessary) and execute clang-format.sh.

EDIT: bonus points for a CI test, that runs in ci-common.git and verifies that clang-format.sh does its job. We can do that with example files, which we put in clang-format/test/valid.c and clang-format/test/invalid.c.