clock: Make it possible to move assign

When the internal Event is const, it makes it impossible to generate the
implicit move constructors since moving an Event is non-const.

Tested:
    Object is now movable by containers. This was verified by using a
    future commit that requires movability of timers.

Change-Id: Id9859806899443e3daf425dacd4bd23fe9a52f58
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/sdeventplus/clock.hpp b/src/sdeventplus/clock.hpp
index b3a7ea9..8af9143 100644
--- a/src/sdeventplus/clock.hpp
+++ b/src/sdeventplus/clock.hpp
@@ -52,7 +52,7 @@
     time_point now() const;
 
   private:
-    const Event event;
+    Event event;
 };
 
 } // namespace sdeventplus