res_rtp_asterisk.c: Add conditional module dependency to res_pjproject

res_rtp_asterisk.c: Add conditional module dependency to res_pjproject

* The dependency ensures that res_pjproject cannot be manually unloaded
before res_rtp_asterisk.
* The dependency allows startup loading errors to report that
res_rtp_asterisk depends upon res_pjproject.

Change-Id: Icf5e7581f4ddd6189929f6174c74dd951f887377