[xna] Fix project file

spine-monogame/example/spine-monogame-example.csproj

@@ -228,11 +228,11 @@
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
     <None Include="app.manifest" />
-    <None Include="..\..\..\..\..\..\..\Library\Frameworks\Mono.framework\External\xbuild\MonoGame\v3.0\Assemblies\DesktopGL\libopenal.1.dylib">
+    <None Include="$(MonoGameInstallDirectory)\MonoGame\v3.0\Assemblies\DesktopGL\libopenal.1.dylib">
       <Link>x64\libopenal.1.dylib</Link>
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
-    <None Include="..\..\..\..\..\..\..\Library\Frameworks\Mono.framework\External\xbuild\MonoGame\v3.0\Assemblies\DesktopGL\libSDL2-2.0.0.dylib">
+    <None Include="$(MonoGameInstallDirectory)\MonoGame\v3.0\Assemblies\DesktopGL\libSDL2-2.0.0.dylib">
       <Link>x64\libSDL2-2.0.0.dylib</Link>
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>