Revert "dont sign assemblies for now."

This reverts commit 386f51a6a3.

build/SharedVersion.props

@@ -15,7 +15,8 @@
     <PackageTags>avalonia;avaloniaui;mvvm;rx;reactive extensions;android;ios;mac;forms;wpf;net;netstandard;net461;uwp;xamarin</PackageTags>
     <PackageReleaseNotes>https://github.com/AvaloniaUI/Avalonia/releases</PackageReleaseNotes>
     <RepositoryType>git</RepositoryType>
-    <AssemblyOriginatorKeyFile>$(MSBuildThisFileDirectory)\avalonia.snk</AssemblyOriginatorKeyFile>    
+    <AssemblyOriginatorKeyFile>$(MSBuildThisFileDirectory)\avalonia.snk</AssemblyOriginatorKeyFile>
+    <SignAssembly>True</SignAssembly>
   </PropertyGroup>
 
   <ItemGroup Label="PackageIcon">