[MacOS] Workaround for 15.6 mac targets issue

Xamarin.Forms.ControlGallery.MacOS/Xamarin.Forms.ControlGallery.MacOS.csproj

@@ -415,4 +415,5 @@
     <InterfaceDefinition Include="Main.storyboard" />
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath)\Xamarin\Mac\Xamarin.Mac.CSharp.targets" />
+  <Target Name="_VerifyBuildSignature" />
 </Project>

Xamarin.Forms.Maps.MacOS/Xamarin.Forms.Maps.macOS.csproj

@@ -94,4 +94,5 @@
     </ProjectReference>
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath)\Xamarin\Mac\Xamarin.Mac.CSharp.targets" />
+  <Target Name="_VerifyBuildSignature" />
 </Project>

Xamarin.Forms.Platform.MacOS/Xamarin.Forms.Platform.macOS.csproj

@@ -242,4 +242,5 @@
     </ProjectReference>
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath)\Xamarin\Mac\Xamarin.Mac.CSharp.targets" />
+  <Target Name="_VerifyBuildSignature" />
 </Project>