nat_traversal: link with lib kcore

- include lib/kcore/statistics.h
- fix undefined symbol for register_module_stats
- reported by Ricardo Martinez
(cherry picked from commit 7c6600c007be55263b48ea1114cbecab86347076)

modules_k/nat_traversal/Makefile

@@ -12,5 +12,8 @@ LIBS=
 
 DEFS+=-DOPENSER_MOD_INTERFACE
 
+SERLIBPATH=../../lib
+SER_LIBS+=$(SERLIBPATH)/kcore/kcore
+
 include ../../Makefile.modules
 

modules_k/nat_traversal/nat_traversal.c

@@ -49,6 +49,7 @@
 #include "../../parser/parse_uri.h"
 #include "../../parser/parse_expires.h"
 #include "../../parser/contact/parse_contact.h"
+#include "../../lib/kcore/statistics.h"
 #include "../dialog/dlg_load.h"
 #include "../../modules/tm/tm_load.h"
 #include "../sl/sl_cb.h"