Fix #6603: PInvokeStackImbalance error caused by incorrect signature for WindowsDeleteString function.

When PreserveSig=false, a function that natively returns an HRESULT and has no final [out] parameter should instead be marked as void.

src/Windows/Avalonia.Win32/WinRT/NativeWinRTMethods.cs

@@ -23,7 +23,7 @@ namespace Avalonia.Win32.WinRT
 
         [DllImport("api-ms-win-core-winrt-string-l1-1-0.dll", 
             CallingConvention = CallingConvention.StdCall, PreserveSig = false)]
-        internal static extern unsafe IntPtr WindowsDeleteString(IntPtr hString);
+        internal static extern unsafe void WindowsDeleteString(IntPtr hString);
         
         [DllImport("Windows.UI.Composition", EntryPoint = "DllGetActivationFactory",
             CallingConvention = CallingConvention.StdCall, PreserveSig = false)]