chan_pjsip.c: Change SSRC after media source change

chan_pjsip.c: Change SSRC after media source change

When the RTP media source changes, such as after a blind transfer, the new source introduces a discontinuous timestamp. According to RFC 3550, Section 5.1, an RTP stream's timestamp for a given SSRC must increment monotonically and linearly.
To comply with the standard and avoid a large timestamp jump on the existing SSRC, a new SSRC is generated for the new media stream.
This change resolves known interoperability issues with certain SBCs (like Sonus/Ribbon) that stop forwarding media when they detect such a timestamp violation. This code uses the existing implementation from chan_sip.

Resolves: #927
(cherry picked from commit 9bef59f44daa0587a19eb8feba8c9d7de25271cd)