include all pj headers in extern "C"

Change-Id: I704857602be0c65751721331707b1a9928f4785b
diff --git a/src/sip_utils.h b/src/sip_utils.h
index 0349b04..9038d8a 100644
--- a/src/sip_utils.h
+++ b/src/sip_utils.h
@@ -24,7 +24,9 @@
 #include <cstring> // strcmp
 #include <memory>
 
+extern "C" {
 #include <pjlib.h>
+}
 
 namespace dhtnet {
 namespace sip_utils {