Add .clang-format-ignore to ignore submodules
Small improvement to clang-format
usage.
Please also mind that we could ignore more extensions to synchronise with formatAll
. Currently, we're using those: https://gitlab.cern.ch/cms_tk_ph2/Ph2_ACF/-/blob/Dev/.gitlab/ci/scripts/run-clang-format.py?ref_type=heads#L34