use System.Runtime instead of mscorlib

src/Shared/HandyControl_Shared/Themes/Styles/Base/TextBoxBaseStyle.xaml

@@ -2,7 +2,7 @@
                     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                     xmlns:hc="clr-namespace:HandyControl.Controls"
                     xmlns:interactivity="clr-namespace:HandyControl.Interactivity"
-                    xmlns:system="clr-namespace:System;assembly=mscorlib"
+                    xmlns:system="clr-namespace:System;assembly=System.Runtime"
                     xmlns:themes="clr-namespace:HandyControl.Themes">
 
     <ResourceDictionary.MergedDictionaries>