Add reference assemblies to support cross-building on mono

Author: hazzik

src/NHibernate.Driver.MySqlConnector/NHibernate.Driver.MySqlConnector.csproj

@@ -18,6 +18,10 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
     <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0">
         <PrivateAssets>all</PrivateAssets>
     </PackageReference>