playerctl: mark as broken on darwin

Build fails with:
../playerctl/playerctl-player.c:274:19: error: use of undeclared identifier 'CLOCK_MONOTONIC'
    clock_gettime(CLOCK_MONOTONIC, &player->priv->cached_position_monotonic);

pkgs/tools/audio/playerctl/default.nix

@@ -20,5 +20,6 @@ stdenv.mkDerivation rec {
     license = licenses.lgpl3;
     platforms = platforms.unix;
     maintainers = with maintainers; [ puffnfresh ];
+    broken = stdenv.hostPlatform.isDarwin;
   };
 }