Use libc's seccomp-related constants

[boustrophedon]

As mentioned in #58 once rust-lang/libc#3343 lands we should use libc's constants instead of redefining them.

https://github.com/rust-vmm/seccompiler/blob/main/src/backend/bpf.rs#L116
and SECCOMP_SET_MODE_FILTER which is being added to lib.rs in #58