Remove isLinux assertion from libcap

Heimdal evaluates libcap's meta.platforms to determine if it should be pulled in or not,
and that fails if the stdenv.isLinux assertion fails.

pkgs/os-specific/linux/libcap/default.nix

···
 { stdenv, fetchurl, attr, perl }:
 
-assert stdenv.isLinux;
-
 stdenv.mkDerivation rec {
   name = "libcap-${version}";
   version = "2.24";