AP_HAL_SITL: use OS clock for get_hw_rtc

libraries/AP_HAL_SITL/Util.cpp

@@ -0,0 +1,10 @@
+#include "Util.h"
+
+uint64_t HALSITL::Util::get_hw_rtc() const
+{
+    struct timespec ts;
+    clock_gettime(CLOCK_REALTIME, &ts);
+    const uint64_t seconds = ts.tv_sec;
+    const uint64_t nanoseconds = ts.tv_nsec;
+    return (seconds * 1000000ULL + nanoseconds/1000ULL);
+}

libraries/AP_HAL_SITL/Util.h

@@ -28,6 +28,9 @@ public:
     const char* get_custom_defaults_file() const override {
         return sitlState->defaults_path;
     }
+
+    uint64_t get_hw_rtc() const override;
+
 private:
     SITL_State *sitlState;
 };