include all pj headers in extern "C"

Change-Id: I704857602be0c65751721331707b1a9928f4785b
diff --git a/src/ice_transport.cpp b/src/ice_transport.cpp
index a257c92..47a67e6 100644
--- a/src/ice_transport.cpp
+++ b/src/ice_transport.cpp
@@ -26,7 +26,9 @@
 #include <opendht/logger.h>
 #include <opendht/utils.h>
 
+extern "C" {
 #include <pjlib.h>
+}
 
 #include <map>
 #include <atomic>