Add targeting pack for linux

Directory.Build.props

@@ -99,6 +99,7 @@
   <!-- Package references which are consumed by all projects -->
   <ItemGroup>
     <PackageReference Include="Microsoft.Net.Compilers.Toolset" IsImplicitlyDefined="true" />
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" IsImplicitlyDefined="true"  />
   </ItemGroup>
 
 </Project>

Directory.Build.targets

@@ -19,6 +19,7 @@
   <ItemGroup>
     <!--Global Dependencies-->
     <PackageReference Update="Microsoft.Net.Compilers.Toolset" Version="3.3.1" />
+    <PackageReference Update="Microsoft.NETFramework.ReferenceAssemblies" PrivateAssets="All" Version="1.0.0" />
 
     <!--Src Dependencies-->
     <PackageReference Update="SixLabors.Core" Version="1.0.0-beta0008" />