pjsip: avoid forward-declare in c++ namespace

src/im/instant_messaging.h

src/ip_utils.cpp

src/ip_utils.h

src/jamidht/connectionmanager.cpp

src/jamidht/connectionmanager.h

src/jamidht/jamiaccount.cpp

src/jamidht/namedirectory.cpp

src/preferences.h

src/sip/sip_utils.h

src/sip/sipaccount.h

src/sip/sipcall.h

src/sip/sippresence.h