Add missing call to ConditionVariableCancelSleep().

After calling ConditionVariableSleep() or ConditionVariableTimedSleep()
one or more times, code is supposed to call ConditionVariableCancelSleep()
to remove itself from the waitlist. This code neglected to do so.
As far as I know, that had no observable consequences, but let's make
the code correct.

Discussion: http://postgr.es/m/CA+TgmoYW8eR+KN6zhVH0sin7QH6AvENqw_bkN-bB4yLYKAnsew@mail.gmail.com

src/backend/postmaster/walsummarizer.c

@@ -682,6 +682,8 @@ WaitForWalSummarization(XLogRecPtr lsn, long timeout, XLogRecPtr *pending_lsn)
 									WAIT_EVENT_WAL_SUMMARY_READY);
 	}
 
+	ConditionVariableCancelSleep();
+
 	return summarized_lsn;
 }