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/event_loop.h b/aos/events/event_loop.h
index c81fa1e..11368ae 100644
--- a/aos/events/event_loop.h
+++ b/aos/events/event_loop.h
@@ -542,8 +542,8 @@
   virtual ~EventLoop();
 
   // Current time.
-  virtual monotonic_clock::time_point monotonic_now() = 0;
-  virtual realtime_clock::time_point realtime_now() = 0;
+  virtual monotonic_clock::time_point monotonic_now() const = 0;
+  virtual realtime_clock::time_point realtime_now() const = 0;
 
   template <typename T>
   const Channel *GetChannel(const std::string_view channel_name) {