Replace Firecraker's seccomp with seccomp-sys

Some distributions require all crate dependencies to be packaged individually. This implies that depending on the whole Firecracker repository would make things very hard for us to get virtiofsd-rs packaged.

This change replaces the dependency on Firecracker's seccomp with seccomp-sys, which is a crate providing raw bindings for libseccomp. This means we add the latter as a both a build time and run time dependency, but that should be fine as the C implementation also depends on it.

I've also considered adding syscallz, which provides safe wrappers on top of syscomp-sys, but that one also depends on strut, which we need to drop from our dependencies due to its problematic versioning policy.

Signed-off-by: Sergio Lopez slp@redhat.com