fcntl F_SEAL not available on FreeBSD

[PolyMeilex]

nix/src/fcntl.rs

Lines 437 to 440 in 1647189

	#[cfg(any(target_os = "android", target_os = "linux"))]
	F_ADD_SEALS(SealFlag),
	#[cfg(any(target_os = "android", target_os = "linux"))]
	F_GET_SEALS,

FreeBSD manual lists those as available, but nix does not.

[asomers]

Yep. Would you care to submit a PR to add it?