add clang-format to CI

- this will run clang-format on all changed C/C++ files
- list of changed files is taken from
https://forum.gitlab.com/t/ci-cd-pipeline-get-list-of-changed-files/26847/18
- loosely based on okular setup
2 jobs for !960 with clang-format-gitlab-ci in 61 minutes and 10 seconds (queued for 4 seconds)
detached
Status Name Job ID Coverage
  Build
failed clang_format #118451

00:01:05

failed linux kf5-qt5 qt5.15 #118450

01:00:04

 
Name Stage Failure
failed
clang_format Build
Setting up python3 (3.9.2-3) ...
Setting up liberror-perl (0.17029-1) ...
Setting up git (1:2.30.2-1) ...
Setting up clang-format-11 (1:11.0.1-2) ...
Processing triggers for libc-bin (2.31-12) ...
$ wget -c --output-document=.clang-format https://invent.kde.org/frameworks/extra-cmake-modules/raw/master/kde-modules/clang-format.cmake
/bin/bash: line 138: wget: command not found
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
linux kf5-qt5 qt5.15 Build The script exceeded the maximum execution time set for the job
147/275 Test #147: libs-image-kis_asl_parser_test ................................   Passed    5.00 sec
Start 148: libs-image-KisPerStrokeRandomSourceTest
148/275 Test #148: libs-image-KisPerStrokeRandomSourceTest ....................... Passed 0.26 sec
Start 149: libs-image-KisWatershedWorkerTest
149/275 Test #149: libs-image-KisWatershedWorkerTest ............................. Passed 1.05 sec
Start 150: libs-image-kis_dom_utils_test
150/275 Test #150: libs-image-kis_dom_utils_test ................................. Passed 0.25 sec
Start 151: libs-image-kis_transform_worker_test
ERROR: Job failed: execution took longer than 1h0m0s seconds