Timer: Make sure timer_on_auto() to use timer_advance_u64() instead of timer_set_delay_u64() if it's called inside a callback.

src/include/86box/timer.h

@@ -43,7 +43,7 @@ typedef struct pc_timer_t {
     ts_t ts;
 #endif
     int    flags;  /* The flags are defined above. */
-    int    pad;
+    int    in_callback;
     double period; /* This is used for large period timers to count
                       the microseconds and split the period. */