ngrep: Make available on darwin

ngrep works on macos/Darwin without problems. The official page even
states that:

```
Confirmed Working Platforms

 Linux 2.0+ (RH6+, SuSE, TurboLinux, Debian, Gentoo, Ubuntu, Mandrake, Slackware)/x86, RedHat/alpha Cobalt, (Qube2) Linux/MIPS
 Solaris 2.5.1, 2.6/SPARC, Solaris 7, Solaris 8/SPARC, Solaris 9/SPARC
 FreeBSD 2.2.5, 3.1, 3.2, 3.4-RC, 3.4-RELEASE, 4.0, 5.0
 OpenBSD 2.4 (after upgrading pcap from 0.2), 2.9, 3.0, 3.1+
 NetBSD 1.5/SPARC
 Digital Unix V4.0D (OSF/1), Tru64 5.0, Tru64 5.1A
 HPUX 11
 IRIX
 AIX 4.3.3.0/PowerPC
 BeOS R5
 Mac OS X 10+
 GNU HURD
 Windows 95, 98, NT, 2000, XP, 2003/x86, 7, 8, 8.1, 10
```

pkgs/tools/networking/ngrep/default.nix

@@ -48,7 +48,7 @@ stdenv.mkDerivation rec {
     # 'BSD-like' license but that the 'regex' library (in the ngrep tarball) is
     # GPLv2.
     license = "ngrep";  # Some custom BSD-style, see LICENSE.txt
-    platforms = platforms.linux;
+    platforms = with platforms; linux ++ darwin;
     maintainers = [ maintainers.bjornfor ];
   };
 }