const-qualify Context::data

Makes it harder for people to accidentally write through it, which they
shouldn't be.

Change-Id: Icea11e9090e01813e3e7b4a15aad601d8b5478c7
diff --git a/aos/events/event_loop.h b/aos/events/event_loop.h
index 81dfe42..b96f25e 100644
--- a/aos/events/event_loop.h
+++ b/aos/events/event_loop.h
@@ -57,7 +57,7 @@
   // Size of the data sent.
   size_t size;
   // Pointer to the data.
-  void *data;
+  const void *data;
 };
 
 // Raw version of fetcher. Contains a local variable that the fetcher will