Mark monotonic_now and realtime_now as const

Change-Id: I054ecc9c3c64b77e3c2bfa452d996b03797fdf64
Signed-off-by: Austin Schuh <austin.schuh@bluerivertech.com>
diff --git a/aos/events/simulated_event_loop.cc b/aos/events/simulated_event_loop.cc
index 2a276e7..a2428a4 100644
--- a/aos/events/simulated_event_loop.cc
+++ b/aos/events/simulated_event_loop.cc
@@ -584,11 +584,11 @@
     send_delay_ = send_delay;
   }
 
-  monotonic_clock::time_point monotonic_now() override {
+  monotonic_clock::time_point monotonic_now() const override {
     return node_event_loop_factory_->monotonic_now();
   }
 
-  realtime_clock::time_point realtime_now() override {
+  realtime_clock::time_point realtime_now() const override {
     return node_event_loop_factory_->realtime_now();
   }