res/res_pjsip_sdp_rtp: put rtcp-mux in answer only if offered

If in the initial sdp the caller doesn't include the line
a=rtcp-mux

Then asterisk shoud not include rtcp-mux in the response regardless
of rtcp-mux being enabled on the endpoint

ASTERISK-28007 #close

Change-Id: I58e9b9f40a139afc0da5de41906cc608fb62adc7