Fix seccomp constants on musl/all linux platforms

[boustrophedon]

on x86_64-unknown-linux-gnu, some of the seccomp-related constants are provided but they are not on all platforms.

These constants live inside <linux/seccomp.h> and it appears that some of them exist in rust's libc linux/mod.rs
Linux: https://github.com/torvalds/linux/blob/master/include/uapi/linux/seccomp.h#L10
Rust libc linux/mod.rs:

libc/src/unix/linux_like/linux/mod.rs

Line 2190 in 8356615

pub const SECCOMP_MODE_DISABLED: ::c_uint = 0;

Rust libc linux/gnu//mod.rs

libc/src/unix/linux_like/linux/gnu/b64/x86_64/mod.rs

Line 805 in 8356615

pub const SECCOMP_SET_MODE_STRICT: ::c_uint = 0;

The constants

/* Valid operations for seccomp syscall. */
#define SECCOMP_SET_MODE_STRICT		0
#define SECCOMP_SET_MODE_FILTER		1
#define SECCOMP_GET_ACTION_AVAIL	2
#define SECCOMP_GET_NOTIF_SIZES		3

from seccomp.h appear to be missing from unix/linux_like/linux/mod.rs . Some other newer constants also appear to be missing e.g. SECCOMP_FILTER_FLAG_NEW_LISTENER.

If it seems okay to move everything into linux/mod.rs, I'll make a PR for that.