AP_HAL: added set_system_clock() API

libraries/AP_HAL/Util.h

@@ -33,6 +33,11 @@ public:
       return state of safety switch, if applicable
      */
     virtual enum safety_state safety_switch_state(void) { return SAFETY_NONE; }
+
+    /*
+      set system clock in UTC microseconds
+     */
+    virtual void set_system_clock(uint64_t time_utc_usec) {}
 };
 
 #endif // __AP_HAL_UTIL_H__