Fix mstimer elapsed time (#58)

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

src/bacnet/basic/sys/mstimer.c

@@ -106,6 +106,21 @@ unsigned long mstimer_remaining(struct mstimer *t)
     return t->start + t->interval - mstimer_now();
 }
 
+/**
+ * The time elapsed since the timer started
+ *
+ * This function returns the time elapsed.
+ *
+ * @param t A pointer to the timer
+ *
+ * @return The time elapsed since the last start of the timer
+ *
+ */
+unsigned long mstimer_elapsed(struct mstimer *t)
+{
+  return mstimer_now() - t->start;
+}
+
 /**
  * The value of the interval
  *