[maccatalyst] Make sure MacProxy is included in System.Net.Http (#91599)

[maccatalyst] Make sure MacProxy is included in System.Net.Http (#91599)

https://github.com/dotnet/runtime/issues/90258 identified that `HttpClient.DefaultProxy` returned `System.Net.Http.HttpNoProxy` when using MacCatalyst. This is due to a condition in System.Net.Http.csproj not including maccatalyst as a condition and as a result, skipped adding the MacProxy sources.

Way back when, https://github.com/dotnet/runtime/pull/47823 had the condition, but this appears to have changed in https://github.com/dotnet/runtime/pull/48652 when we thought MacCatalyst would inherit settings from iOS. That turned out to not be the case and this check was never added back.

Co-authored-by: Steve Pfister <steve.pfister@microsoft.com>