Fix DLGPROC prototype

src/arch/LoadingWindow/LoadingWindow_Win32.cpp

@@ -81,7 +81,7 @@ static HBITMAP LoadWin32Surface( RString sFile, HWND hWnd )
         return ret;
 }
 
-BOOL CALLBACK LoadingWindow_Win32::WndProc( HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam )
+INT_PTR CALLBACK LoadingWindow_Win32::WndProc( HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam )
 {
 	switch( msg )
 	{