plugins/elements/gstmultiqueue.c: When dynamically creating single queues, activate...

plugins/elements/gstmultiqueue.c: When dynamically creating single queues, activate sinkpad before adding it.

Original commit message from CVS:
* plugins/elements/gstmultiqueue.c: (gst_single_queue_new):
When dynamically creating single queues, activate sinkpad before adding
it.
We should be doing the same thing for the source pad, but we can't
since it would call a method which needs the parent to be set in order
to work propertly. Instead of activating the source pad, we just unset
the flushing flag, which is the minimal requirement for adding a pad
to an element in a state greater than READY.