Fixes these compiler warnings:
xamarin-macios/src/ObjCRuntime/ErrorHelper.cs(22,17): warning CS0436: The type 'ProductException' in 'xamarin-macios/tests/msbuild/Xamarin.MacDev.Tasks.Tests/../../../tools/common/error.cs' conflicts with the imported type 'ProductException' in 'Xamarin.iOS.Tasks, Version=15.11.0.465, Culture=neutral, PublicKeyToken=84e04ff9cfb79065'. Using the type defined in 'xamarin-macios/tests/msbuild/Xamarin.MacDev.Tasks.Tests/../../../tools/common/error.cs'. [xamarin-macios/tests/msbuild/Xamarin.MacDev.Tasks.Tests/Xamarin.MacDev.Tasks.Tests.csproj]
xamarin-macios/tools/common/error.cs(10,34): warning CS0436: The type 'ErrorHelper' in 'xamarin-macios/tests/msbuild/Xamarin.MacDev.Tasks.Tests/../../common/ErrorHelper.tests.cs' conflicts with the imported type 'ErrorHelper' in 'Xamarin.iOS.Tasks, Version=15.11.0.465, Culture=neutral, PublicKeyToken=84e04ff9cfb79065'. Using the type defined in 'xamarin-macios/tests/msbuild/Xamarin.MacDev.Tasks.Tests/../../common/ErrorHelper.tests.cs'. [xamarin-macios/tests/msbuild/Xamarin.MacDev.Tasks.Tests/Xamarin.MacDev.Tasks.Tests.csproj]