gst/gstbin.c: Take the (recursive) state lock between getting the locked state of an element and changing the element...

Original commit message from CVS:
* gst/gstbin.c: (gst_bin_element_set_state):
Take the (recursive) state lock between getting the locked state of an
element and changing the element state. This allows the application to
lock an element's state and then change its state without races.