Make SourceLink a development dependency only

From https://github.com/dotnet/sourcelink#githubcom-and-github-enterprise
> Source Link package is a development dependency, which means it is only used during build. It is therefore recommended to set PrivateAssets to all on the package reference. This prevents consuming projects of your nuget package from attempting to install Source Link.

Author: jnyrup

RestSharp/RestSharp.csproj

@@ -24,7 +24,7 @@
     <Service Include="{508349b6-6b84-4df5-91f0-309beebad82d}" />
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0-beta2-18618-05" />
+    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0-beta2-18618-05" PrivateAssets="All" />
   </ItemGroup>
   <PropertyGroup>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>