res_pjsip_session: Always bundle streams if WebRTC is enabled.

Some WebRTC clients can't handle renegotiation with the addition of
streams that include an offer to bundle. They instead expect the
newly added streams to already be bundled. This change does such a thing
if WebRTC support is enabled on an endpoint.

ASTERISK-27566

Change-Id: I7fe9b7ac35a2798627d9c2c8369129f407af6461