I have this running on Fedora 35. I ran into the same issue at first, but the key is this line right here:
extlibs/gameswf/gameswf/gameswf_environment.h:51:16: note: declared private here
51 | struct vm_stack : private array<as_value>
If you go into that file: extlibs/gameswf/gameswf/gameswf_environment.h
Line 51: struct vm_stack : private array<as_value>
and change private to public, it will compile without issue.