Wake up interested backends when a checkpoint fails.

Commit c6c9474a switched to condition variables instead of sleep
loops to notify backends of checkpoint start and stop, but forgot
to broadcast in case of checkpoint failure.

Author: Thomas Munro
Discussion: https://postgr.es/m/CA%2BhUKGJKbCd%2B_K%2BSEBsbHxVT60SG0ivWHHAdvL0bLTUt2xpA2w%40mail.gmail.com

src/backend/postmaster/checkpointer.c

@@ -279,6 +279,8 @@ CheckpointerMain(void)
 			CheckpointerShmem->ckpt_done = CheckpointerShmem->ckpt_started;
 			SpinLockRelease(&CheckpointerShmem->ckpt_lck);
 
+			ConditionVariableBroadcast(&CheckpointerShmem->done_cv);
+
 			ckpt_active = false;
 		}