Add Microsoft.NETFramework.ReferenceAssemblies for cross-platform pack

Author: lilith

src/Imazen.WebP/Imazen.WebP.csproj

@@ -11,6 +11,11 @@
     <PackageReadmeFile Condition="Exists('$(MSBuildThisFileDirectory)..\..\README.md')">README.md</PackageReadmeFile>
   </PropertyGroup>
 
+  <!-- Reference assemblies for cross-platform builds of .NET Framework targets -->
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.3" PrivateAssets="all" />
+  </ItemGroup>
+
   <!-- System.Drawing.Common for netstandard2.0 and net8.0 -->
   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0' Or '$(TargetFramework)' == 'net8.0'">
     <PackageReference Include="System.Drawing.Common" Version="8.*" />