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.