Fix libavutil ffversion.h header generation
Properly escape all invalid C-macro characters of header guard. Fixes compilation with meson as subproject using non-default subproject_dir like: '3rdParty', or wrap name 'ffmpeg-git.wrap'
In such case generated guard name would be: 3RDPARTY_FFMPEG-GIT_AVUTIL_FFVERSION_H
After this change: _3RDPARTY_FFMPEG_GIT_AVUTIL_FFVERSION_H