Build with documentation fails (--build-doxygen option) due to missing file

When trying to build the documentation with doxygen (e.g. with: ./cmake_targets/build_oai -w USRP -x -c --eNB --build-doxygen), the build fails due to a missing file targets/DOCS/openair_header.tex. The file is a LaTeX header. It was deleted in this commit https://github.com/brchiu/openairinterface5g/commit/7e58b8610ec0394390664ebf4ed8a1aeb74061e4 , together with some other files. Probably, this commit cleaned up the sources a bit too much. When restoring the missing file, I was able to build the documentation.