[#2647] Start zrtp engin when receiving rtp packet only

There is a mechanism in libzrtpcpp that start the library as soon as RTP packet are received.
Using this mechanism (i.e. not calling zrtpStart() when initializing the library) fix the issue.

sflphone-common/src/audio/audiortp/AudioZrtpSession.cpp

@@ -38,7 +38,7 @@ AudioZrtpSession::AudioZrtpSession (ManagerImpl * manager, SIPCall * sipcall, co
 {
     _debug ("AudioZrtpSession initialized");
     initializeZid();
-    startZrtp();
+    // startZrtp();
 }
 
 void AudioZrtpSession::initializeZid (void)