Fix credential event callback reference lifetime

src/Lithnet.CredentialProvider.UnitTests.x64/ComInterop/RawQueryInterfaceDelegate.cs

@@ -0,0 +1,8 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace Lithnet.CredentialProvider.UnitTests.ComInterop
+{
+    [UnmanagedFunctionPointer(CallingConvention.StdCall)]
+    internal delegate int RawQueryInterfaceDelegate(IntPtr instance, ref Guid interfaceId, out IntPtr interfacePointer);
+}