res_pjsip_sdp_rtp.c: Don't use deprecated transport struct member.

* create_rtp(): Eliminate use of deprecated transport struct member.  That
member and several others in the transport structure were deprecated
because of an infinite loop created when using realtime configuration.
See 2451d4e4

ASTERISK-26851

Change-Id: I0533aa13c9ce3c6cc394e0fd2b5bf1cd1b2ef3bc