Fix build errors from new examples

Author: mikekistler

src/OpenApi/src/Microsoft.AspNetCore.OpenApi.csproj

@@ -20,6 +20,7 @@
     <Reference Include="Microsoft.AspNetCore.Routing" />
     <Reference Include="Microsoft.AspNetCore.Mvc.Core" />
     <Reference Include="Microsoft.AspNetCore.Mvc.ApiExplorer" />
+    <Reference Include="Microsoft.AspNetCore.DefaultBuilder" />
   </ItemGroup>
 
   <ItemGroup>