res_rtp_asterisk: Fix wrongful use of USE_PJPROJECT define.

As pjproject is now used as a shared library a different define,
HAVE_PJPROJECT, is used to specify if pjproject is present.

ASTERISK-24830 #close
Reported by: Stefan Engström
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Merged revisions 432614 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432615 65c4cc65-6c06-0410-ace0-fbb531ad65f3