Replace header guard in ffversion.h with #pragma once
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, it will be protected by #pragma once
Edited by Piotr Rak