mpd: fix 0.23.8 on darwin

- 0.23.8 includes c975d8b943
- said change fixes deprecation warnings introduced in the 12.0 sdk, using definitions from the 12.0 sdk
- nixpkgs uses 10.12 and 11.0, neither of which are the 12.0 sdk
- this will gracefully degrade into a no-op when that changes

pkgs/servers/mpd/default.nix

@@ -145,6 +145,12 @@ let
       ]
         ++ concatAttrVals features_ nativeFeatureDependencies;
 
+      postPatch = lib.optionalString (stdenv.isDarwin && lib.versionOlder stdenv.targetPlatform.darwinSdkVersion "12.0") ''
+        substituteInPlace src/output/plugins/OSXOutputPlugin.cxx \
+          --replace kAudioObjectPropertyElement{Main,Master} \
+          --replace kAudioHardwareServiceDeviceProperty_Virtual{Main,Master}Volume
+      '';
+
       # Otherwise, the meson log says:
       #
       #    Program zip found: NO