Fix missing wake() in PendingOnce #2372

Job	Run time
cargo bench	51s
cargo clippy	40s
cargo +stable build	36s
codegen	24s
cargo +beta build	43s
cargo +1.36 build (futures-{core, io, sink})	4m 31s
cargo +nightly build	40s
cargo hack check --feature-powerset	4m 8s
cargo minimal-versions build	36s
cargo fmt	4s
cargo doc	29s
cargo miri test	7m 17s
cargo build --target thumbv7m-none-eabi	32s
cargo test -Z sanitizer=address	2m 51s
cargo build --target thumbv6m-none-eabi	27s
cargo test (ubuntu-latest)	3m 3s
cargo +1.56 build	1m 50s
cargo test -Z sanitizer=memory	3m 39s
cargo test -Z sanitizer=thread	3m 12s
cargo test (macos-latest)	4m 16s
cargo test (windows-latest)	4m 19s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 22s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 4s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	8m 21s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	5m 26s
	1h 8m 21s

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