Add rustfmt skip directives to channelmanager.rs

This allows devs to enable rustfmt on a per-function level.

Author: joostjager

lightning/src/ln/channelmanager.rs

@@ -11,7 +11,6 @@ lightning/src/ln/blinded_payment_tests.rs
 lightning/src/ln/chan_utils.rs
 lightning/src/ln/chanmon_update_fail_tests.rs
 lightning/src/ln/channel.rs
-lightning/src/ln/channelmanager.rs
 lightning/src/ln/functional_test_utils.rs
 lightning/src/ln/max_payment_path_len_tests.rs
 lightning/src/ln/mod.rs