[XM] Fix mmp test failure

- On some machines/mono versions it appears that printing System.Windows.DependencyObject requires System.Xaml for some reason.
- Since the test is testing our resolution from the GAC and we don't care about the specifics, I'm just adding that reference. Which we also get from the GAC.

tests/mmptest/src/MMPTest.cs

@@ -315,7 +315,7 @@ namespace Xamarin.MMP.Tests
 				{
 					XM45 = true,
 					TestCode = "System.Console.WriteLine (typeof (System.Windows.DependencyObject));",
-					References = "<Reference Include=\"WindowsBase\" />"
+					References = "<Reference Include=\"WindowsBase\" /><Reference Include=\"System.Xaml\" />"
 				};
 
 				TI.TestUnifiedExecutable (test, shouldFail : true);