(trivial) xwin: fix memleak on freeing pixmaps
Xwin's DestroyPixmap proc just free()s the PixmapRec directly, w/o catering for devPrivate's, so leaving a memleak. The correct DIX function for this is FreePixmap().
Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net