Added empty function for ns time callback

Author: Mika Leppänen

source/6LoWPAN/ws/ws_empty_functions.c

@@ -25,6 +25,7 @@
 #include "6LoWPAN/ws/ws_common.h"
 
 #include "ws_management_api.h"
+#include "ns_time_api.h"
 
 #ifndef HAVE_WS
 int ws_management_node_init(
@@ -392,4 +393,9 @@ int ws_statistics_stop(int8_t interface_id)
     return -1;
 }
 
+void ns_time_api_system_time_callback_set(ns_time_api_system_time_callback callback)
+{
+    (void) callback;
+}
+
 #endif // no HAVE_WS