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/shm_event_loop.h b/aos/events/shm_event_loop.h
index 71dff4e..e51f21b 100644
--- a/aos/events/shm_event_loop.h
+++ b/aos/events/shm_event_loop.h
@@ -48,10 +48,10 @@
   // Exits the event loop.  Async safe.
   void Exit();
 
-  aos::monotonic_clock::time_point monotonic_now() override {
+  aos::monotonic_clock::time_point monotonic_now() const override {
     return aos::monotonic_clock::now();
   }
-  aos::realtime_clock::time_point realtime_now() override {
+  aos::realtime_clock::time_point realtime_now() const override {
     return aos::realtime_clock::now();
   }