GdkWin32: fix leak of event structures

In gdkevents-win32.c: generate_button_event we have this code:

static void
generate_button_event ()
{
  GdkEvent *event = gdk_event_new (type);
  /* ... */

  if (_gdk_input_ignore_core > 0)
    return;

If gdk_input_ignore_core is > 0 we leak an event structure. Fix that.