fixed xamlns clr-namespace: ... not working on uwp

[XiaotianNetlift]

Issue: #64

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

Uwp fails with:

C:\code\Uno.WindowsCommunityToolkit\Microsoft.Toolkit.Uwp.SampleApp.Shared\Pages\SampleController.xaml(343,26): 
XamlCompiler error WMC0001: Unknown type 'PaneStateTrigger' in XML namespace 
'clr-namespace:Microsoft.Toolkit.Uwp.SampleApp.Shared.Triggers;assembly=Microsoft.Toolkit.Uwp.SampleApp, Version=5.1.0.0, Culture=neutral, PublicKeyToken=null'

What is the new behavior?

Uwp build should pass.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

[XiaotianNetlift]

/azp run