gst/gst.defs: Add declaration of gst_object_set_property so we can use our MT-safe...

gst/gst.defs: Add declaration of gst_object_set_property so we can use our MT-safe version of set_property().

Original commit message from CVS:
* gst/gst.defs:
Add declaration of gst_object_set_property so we can use our MT-safe
version of set_property().
* gst/gstobject.override:
Implement a MT-safe version of g_object_set_property for GstObject.
The problem is that currently g_object_set_property is called in
pygobject with the GIL lock taken. This can cause deadlocks.
Remove this hack once bug #395048 is fixed in pygobject and we depend on
the fixed version.
Thanks to Lord Wingo of the "realm.py haters club" for proposing the
idea.