sdp.cpp : fix a warning

sflphone-common/src/sip/sdp.cpp

@@ -232,7 +232,7 @@ void Sdp::setMediaDescriptorLine (sdpMedia *media, pjmedia_sdp_media** p_med)
             rtpmap.clock_rate = codec->getClockRate();
         }
 
-        rtpmap.param.ptr = "";
+        rtpmap.param.ptr = ((char* const)"");
         rtpmap.param.slen = 0;
 
         pjmedia_sdp_rtpmap_to_attr (memPool_, &rtpmap, &attr);