Don't produce symbols for shared sources packages

Author: Nate McMaster

src/Shared/src/Directory.Build.props

@@ -3,6 +3,7 @@
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory)..\, Directory.Build.props))\Directory.Build.props" />
 
   <PropertyGroup>
+    <IncludeSymbols>false</IncludeSymbols>
     <PackageId>Microsoft.Extensions.$(ProjectDirName).Sources</PackageId>
   </PropertyGroup>
 </Project>