multi: add custom message overrides to protocol config options

lncfg/protocol_experimental_off.go

@@ -7,3 +7,9 @@ package lncfg
 // features that also require a build-tag to activate.
 type ExperimentalProtocol struct {
 }
+
+// CustomMessageOverrides returns the set of protocol messages that we override
+// to allow custom handling.
+func (p ExperimentalProtocol) CustomMessageOverrides() []uint16 {
+	return nil
+}