OxyPlot.Xamarin.Mac: add reference to netstandard

* trying to fix errors like this in GHA:
  error CS0012: The type 'ValueType' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'.

Author: janusw

Source/OxyPlot.Xamarin.Mac/OxyPlot.Xamarin.Mac.csproj

@@ -68,6 +68,7 @@
   <ItemGroup>
     <Reference Include="System" />
     <Reference Include="System.Core" />
+    <Reference Include="netstandard" />
   </ItemGroup>
   <ItemGroup Condition="'$(OS)' != 'Windows_NT'">
     <Reference Include="Xamarin.Mac" />