darkstat: Fix build on darwin

On recent macOS the build fails with:
```
now.c:33:16: error: typedef redefinition with different types ('int' vs 'enum clockid_t')
typedef int clockid_t;
^
/nix/store/4df4jhlj71vblmxhrr7dkkqpak8a42c8-Libsystem-1238.60.2/include/time.h:171:3: note: previous definition is here
} clockid_t;
^
now.c:34:11: warning: 'CLOCK_REALTIME' macro redefined [-Wmacro-redefined]
^
/nix/store/4df4jhlj71vblmxhrr7dkkqpak8a42c8-Libsystem-1238.60.2/include/time.h:154:9: note: previous definition is here
^
now.c:35:11: warning: 'CLOCK_MONOTONIC' macro redefined [-Wmacro-redefined]
^
/nix/store/4df4jhlj71vblmxhrr7dkkqpak8a42c8-Libsystem-1238.60.2/include/time.h:156:9: note: previous definition is here
^
2 warnings and 1 error generated.
```

pkgs/tools/networking/darkstat/default.nix

···
-{ lib, stdenv, fetchurl, libpcap, zlib }:
+{ lib, stdenv, fetchpatch, fetchurl, libpcap, zlib }:
 
 stdenv.mkDerivation rec {
   version = "3.0.719";
···
     url = "${meta.homepage}/${pname}-${version}.tar.bz2";
     sha256 = "1mzddlim6dhd7jhr4smh0n2fa511nvyjhlx76b03vx7phnar1bxf";
   };
+
+  patches = [
+    # Avoid multiple definitions of CLOCK_REALTIME on macOS 11,
+    # see https://github.com/emikulic/darkstat/pull/2
+    (fetchpatch {
+       url = "https://github.com/emikulic/darkstat/commit/d2fd232e1167dee6e7a2d88b9ab7acf2a129f697.diff";
+       sha256 = "0z5mpyc0q65qb6cn4xcrxl0vx21d8ibzaam5kjyrcw4icd8yg4jb";
+    })
+  ];
 
   buildInputs = [ libpcap zlib ];