Enable WinMDExp task on .NET Core MSBuild

This task will fail at runtime unless the .NET Framework
SDK is installed, since winmdexp.exe is only present in
that SDK. However, nothing about the task itself is
incompatible with .NET Core.

See also microsoft/microsoft-ui-xaml#1891.
Contributes to dotnet/sdk#3042.

src/Tasks/Microsoft.Build.Tasks.csproj

@@ -481,6 +481,7 @@
     <Compile Include="RoslynCodeTaskFactory\RoslynCodeTaskFactoryCodeType.cs" />
     <Compile Include="RoslynCodeTaskFactory\RoslynCodeTaskFactoryCompilers.cs" />
     <Compile Include="RoslynCodeTaskFactory\RoslynCodeTaskFactoryTaskInfo.cs" />
+    <Compile Include="SdkToolsPathUtility.cs" />
     <Compile Include="System.Design.cs" />
     <Compile Include="system.design\stronglytypedresourcebuilder.cs" />
     <Compile Include="TaskExtension.cs">
@@ -500,6 +501,7 @@
     <Compile Include="Warning.cs">
       <ExcludeFromStyleCop>true</ExcludeFromStyleCop>
     </Compile>
+    <Compile Include="WinMDExp.cs" />
     <Compile Include="XslTransformation.cs" />
     <Compile Include="AssignCulture.cs" />
     <Compile Include="Culture.cs" />
@@ -623,7 +625,6 @@
       <ExcludeFromStyleCop>true</ExcludeFromStyleCop>
     </Compile>
     <Compile Include="ResolveSDKReference.cs" />
-    <Compile Include="SdkToolsPathUtility.cs" />
     <Compile Include="RequiresFramework35SP1Assembly.cs" Condition="'$(MonoBuild)' != 'true'">
       <ExcludeFromStyleCop>true</ExcludeFromStyleCop>
     </Compile>
@@ -649,7 +650,6 @@
     <Compile Include="UpdateManifest.cs" Condition="'$(MonoBuild)' != 'true'">
       <ExcludeFromStyleCop>true</ExcludeFromStyleCop>
     </Compile>
-    <Compile Include="WinMDExp.cs" />
   </ItemGroup>
   <ItemGroup Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework'">
     <Compile Include="XamlTaskFactory\CommandLineGenerator.cs" />