Fix missing Microsoft.NETFramework.ReferenceAssemblies reference

Author: ghorsington

BepInEx.NetLauncher.Common/BepInEx.NetLauncher.Common.csproj

@@ -1,11 +1,12 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
     <PropertyGroup>
         <TargetFrameworks>netstandard2.1;net40</TargetFrameworks>
     </PropertyGroup>
 
     <ItemGroup>
         <PackageReference Include="HarmonyX" Version="2.10.0"/>
+        <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.2" PrivateAssets="All"/>
     </ItemGroup>
 
     <ItemGroup>