Introduce 'seccomp' feature

Cross-compilation of the crate and creation of statically linked binaries based off of it are complicated by the fact that it links against system libraries not written in Rust, seccomp & capng. In the case of seccomp, it really should be left to clients to decide whether the functionality is needed in the first place, at least when consuming the library. It is already only contained in and exposed via a single module, usage of which is entirely optional. To give clients control over inclusion of seccomp features, introduce the 'seccomp' feature, guarding availability of the module of the same name. When disabled, libseccomp won't be linked.

Signed-off-by: Daniel Müller deso@posteo.net