Fix CI failure to fail when a public method is parameterized by a private item

[valentinewallace]

Found on @alecchendev's branch https://github.com/alecchendev/rust-lightning/tree/custom-fail-back-err as part of #1948, commit 60bce30.

The public method ChannelManager::fail_htlc_backwards_with_reason was added and parameterized by HTLCFailReason, and CI passed even though HTLCFailReason is in a private module