#46 Update dtls_time.h (Contiki 3.0 only)

[rfuentess]

Hello, in the file dtls_time.h Lines 46-49 have the following:

#ifdef WITH_CONTIKI
#include "clock.h"
#else /* WITH_CONTIKI */
#include <time.h>

However, for Contiki 3.0 I had to do the following small changes:

#ifdef WITH_CONTIKI 
#include "sys/clock.h"
#else /* WITH_CONTIKI */
#include <time.h>

This is a changed from the way Contiki 2.6 and Contiki 3.0 have their libs.