Enable `TreatWarningsAsErrors` flag is project file

Jellyfin.Drawing.Skia/Jellyfin.Drawing.Skia.csproj

@@ -4,6 +4,7 @@
     <TargetFramework>netstandard2.1</TargetFramework>
     <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>
 
   <ItemGroup>