Add missing test method to ws_empty_functions

Add ws_test_6lowpan_fragmentation_mtu_size_set to ws_empty_functions
to get other configurations compiled.

Author: Arto Kinnunen

source/6LoWPAN/ws/ws_empty_functions.c

@@ -380,6 +380,14 @@ int ws_test_next_gtk_set(int8_t interface_id, uint8_t *gtk[4])
     return -1;
 }
 
+int ws_test_6lowpan_fragmentation_mtu_size_set(int8_t interface_id, uint16_t mtu_size)
+{
+    (void) interface_id;
+    (void) mtu_size;
+
+    return -1;
+}
+
 int ws_statistics_start(int8_t interface_id, ws_statistics_t *stats_ptr)
 {
     (void) interface_id;