Added empty function for test

Author: Mika Tervonen

source/6LoWPAN/ws/ws_empty_functions.c

@@ -138,6 +138,12 @@ int ws_test_pan_size_set(int8_t interface_id, uint16_t pan_size)
     (void) pan_size;
     return -1;
 }
+int ws_test_max_child_count_set(int8_t interface_id, uint16_t child_count)
+{
+    (void) interface_id;
+    (void) child_count;
+    return -1;
+}
 
 #endif // no HAVE_WS