netbsd.librt: fix build
netbsd.librt: fix build

This is another one of those packages that's just a rebuild of a
certain part of libc...

Tested by building pkgsCross.x86_64-netbsd.netbsd.librt on
x86_64-linux.

libc's postPatch was entirely fixes for librt, so move that to librt
and inherit it in libc.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs