[window] Fix a crash in MxWindow due to bad callback prototype
[window] Fix a crash in MxWindow due to bad callback prototype

When changing the style after creating a MxWindow, style_changed_cb is called.
This function had the wrong prototype, leading to crash.

pwithnall/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx