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

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchurl, libpcap, zlib }:
+{ lib, stdenv, fetchpatch, fetchurl, libpcap, zlib }:
 
 stdenv.mkDerivation rec {
   version = "3.0.719";
@@ -9,6 +9,15 @@ stdenv.mkDerivation rec {
     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 ];
 
   enableParallelBuilding = true;