[Xamarin.Android.Build.Tasks] Typo in Xamarin.Android.Windows.targets (#970)

Commit 286b9c28 added extra item to `@(_FilesToRegister)` as part of
the `IncrementalClean` target.

Unfortunately, one of the values omitted the `$`, resulting in an
"invalid" property name reference.

This commit fixes that issue.

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Common/ImportAfter/Xamarin.Android.Windows.targets

@@ -14,7 +14,7 @@ Copyright (C) 2014 Xamarin. All rights reserved.
 	</PropertyGroup>
 
 	<Target Name="_RegisterMdbFilesWithFileWrites" BeforeTargets="IncrementalClean">  
-		<CreateItem Include="$(OutDir)*.dll.mdb;$(MonoAndroidIntermediateAssemblyDir)*.dll.mdb;(MonoAndroidIntermediateAssemblyDir)*.pdb;$(MonoAndroidLinkerInputDir)*.dll.mdb;$(MonoAndroidLinkerInputDir)*.pdb">  
+		<CreateItem Include="$(OutDir)*.dll.mdb;$(MonoAndroidIntermediateAssemblyDir)*.dll.mdb;$(MonoAndroidIntermediateAssemblyDir)*.pdb;$(MonoAndroidLinkerInputDir)*.dll.mdb;$(MonoAndroidLinkerInputDir)*.pdb">  
 			<Output TaskParameter="Include" ItemName="_FilesToRegister" />  
 		</CreateItem>  
 		<CreateItem Include="$([System.IO.Path]::GetFullPath('%(_FilesToRegister.Identity)'))"