common/async/co_waiter: Add missing header

Didn't include `associated_cancellation_slot.hpp`.

Signed-off-by: Adam C. Emerson <[email protected]>

Author: adamemerson

src/common/async/co_waiter.h

@@ -14,6 +14,7 @@
 
 #include <exception>
 #include <optional>
+#include <boost/asio/associated_cancellation_slot.hpp>
 #include <boost/asio/append.hpp>
 #include <boost/asio/async_result.hpp>
 #include <boost/asio/dispatch.hpp>