Re-add Xamarin.Designer.dll

src/Xamarin.iOSProviders/Xamarin.UIProvider.DesignTime.fsproj

@@ -46,6 +46,9 @@
     <Reference Include="MonoTouch.Design">
       <HintPath>/Applications/Visual Studio.app/Contents/Resources/lib/monodevelop/AddIns/MonoDevelop.IPhone/MonoTouch.Design.dll</HintPath>
     </Reference>
+    <Reference Include="Xamarin.Designer">
+      <HintPath>/Applications/Visual Studio.app/Contents/Resources/lib/monodevelop/AddIns/Xamarin.Ide/Xamarin.Designer.dll</HintPath>
+    </Reference>
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath32)\..\Microsoft SDKs\F#\3.0\Framework\v4.0\Microsoft.FSharp.Targets" />
   <ItemGroup>
@@ -102,4 +105,4 @@
       </ItemGroup>
     </When>
   </Choose>
-</Project>
+</Project>