chan_pjsip: Add Referred-By header to the PJSIP REFER packet.

Some systems require the REFER packet to include a Referred-By header.
If the channel variable SIPREFERREDBYHDR is set, it passes that value as the
Referred-By header value.  Otherwise, it adds the current dialog’s local info.

Reported by: Dan Cropp
Tested by: Dan Cropp

Change-Id: I3d17912ce548667edf53cb549e88a25475eda245