Mark Nullable as a private dependency

Author: nblumhardt

src/Serilog.Expressions/Serilog.Expressions.csproj

@@ -18,7 +18,7 @@
 
   <ItemGroup>
     <PackageReference Include="Serilog" Version="2.10.0" />
-    <PackageReference Include="Nullable" Version="1.3.0" Condition="$(TargetFramework) == 'netstandard2.0'" />
+    <PackageReference Include="Nullable" Version="1.3.0" PrivateAssets="All" />
   </ItemGroup>
 
   <ItemGroup>