drv_hrt: inline hrt_elapsed_time

GCC did not do that.

platforms/posix/src/px4_layer/drv_hrt.cpp

@@ -198,19 +198,6 @@ hrt_abstime ts_to_abstime(const struct timespec *ts)
 	return result;
 }
 
-/*
- * Compute the delta between a timestamp taken in the past
- * and now.
- *
- * This function is safe to use even if the timestamp is updated
- * by an interrupt during execution.
- */
-hrt_abstime hrt_elapsed_time(const hrt_abstime *then)
-{
-	hrt_abstime delta = hrt_absolute_time() - *then;
-	return delta;
-}
-
 /*
  * Compute the delta between a timestamp taken in the past
  * and now.