dolphinEmuMaster: mark broken on darwin

The default version isn't enabled on darwin either, however it did work
at some point.

    /tmp/nix-build-dolphin-emu-2018-08-17.drv-0/source/Source/Core/Core/NetPlayServer.cpp:1180:26: error: unknown type name 'lzo_uint32_t'; did you mean 'lzo_uint32'?
      std::vector<u8> wrkmem(LZO1X_1_MEM_COMPRESS);

/cc ZHF #45961

pkgs/misc/emulators/dolphin-emu/master.nix

@@ -78,6 +78,7 @@ in stdenv.mkDerivation rec {
     branch = "master";
     # x86_32 is an unsupported platform.
     # Enable generic build if you really want a JIT-less binary.
+    broken = stdenv.isDarwin;
     platforms = [ "x86_64-linux" "x86_64-darwin" ];
   };
 }