Add NuGet package for System.Runtime.Serialization.Schema (dotnet#74533)

System.Runtime.Serialization.Schema isn't getting a NuGet package created for it because it doesn't set IsPackable=true.

Author: eerhardt

src/libraries/System.Runtime.Serialization.Schema/src/System.Runtime.Serialization.Schema.csproj

@@ -2,6 +2,7 @@
   <PropertyGroup>
     <TargetFramework>$(NetCoreAppCurrent)</TargetFramework>
     <StrongNameKeyId>Microsoft</StrongNameKeyId>
+    <IsPackable>true</IsPackable>
     <!-- TODO: Remove this setting when the package shipped with .NET 7. -->
     <DisablePackageBaselineValidation>true</DisablePackageBaselineValidation>
     <PackageDescription>Provides support for importing and exporting xsd schemas for DataContractSerializer.
@@ -25,4 +26,4 @@ System.Runtime.Serialization.Schema.XsdDataContractImporter</PackageDescription>
   <ItemGroup>
     <ProjectReference Include="$(LibrariesProjectRoot)System.CodeDom\src\System.CodeDom.csproj" />
   </ItemGroup>
-</Project>
+</Project>