AP_Stats: add and use an AP_RTC_config.h

libraries/AP_Stats/AP_Stats.cpp

@@ -108,6 +108,7 @@ void AP_Stats::update()
         params.flttime.set_and_save_ifchanged(0);
         params.runtime.set_and_save_ifchanged(0);
         uint32_t system_clock = 0; // in seconds
+#if AP_RTC_ENABLED
         uint64_t rtc_clock_us;
         if (AP::rtc().get_utc_usec(rtc_clock_us)) {
             system_clock = rtc_clock_us / 1000000;
@@ -115,6 +116,7 @@ void AP_Stats::update()
             // time base to Jan 1st 2016:
             system_clock -= 1451606400;
         }
+#endif
         params.reset.set_and_save_ifchanged(system_clock);
         copy_variables_from_parameters();
     }