Make asterisk13 port use the bundled pjproject(pjsip) library, as
suggested by upstream.

Force the bundled pjproject to use external srtp library provided
by ports to ensure it and asterisk itself link to the same library.

This decouples asterisk from the pjsip port, allowing it to be
upsdaated without breaking asterisk. [1]

PR: 245607 [1]
Submitted by: yuri