seccomp: move sched_getaffinity() from @system-service to @default
    
    See: https://github.com/systemd/systemd/pull/20191#issuecomment-881982739
    
    In general, we shouldn't blanket move syscalls like this into @default,
    given that glibc actually does have fallbacks, afaics. However, as
    long as the syscalls are "read-only" and thus benign, I figure it's a
    safe thing to do. But we should probably stick to a "if in doubt, don't"
    rule, and put these syscalls in @system-service as default, but not into
    @default.
    
    I think in the real world @system-service is the sensible group people
    should use, and not @default actually.
    
    (cherry picked from commit 7df660e45682af5c40a236abe1bdc5ddcf3b3533)