mctp: Bump mctp-lib to branch main

The pinned 9e-buildup branch does not carry the mctp_control module,
which provides the datatypes and the MctpCodec trait for encoding and
decoding DSP0236 control-protocol messages. Branch main carries it, so a
control-protocol responder can be built against this dependency.

Branch main also accepts a packet addressed to EID 0, which 9e-buildup
dropped whenever this endpoint had another EID. An endpoint answers on
EID 0 until a bus owner assigns it one, so that drop left Set Endpoint
ID unanswerable.

Two API changes come with the branch. Sender::send_vectored takes the
destination EID ahead of the fragmenter, which the one production sender
and the five test mocks now accept and ignore: the production sender
routes to the single peer address fixed at construction, and each mock
captures or discards every packet. Router::inbound returns the cookie of
the handle a matched message was queued for, or None for a discarded
one, and Server::inbound drops that value, since Server::update reaches
the message by polling each outstanding handle by cookie.

The mctp pin moves from a floating branch to the rev mctp-lib main
itself pins, so one mctp version resolves for both.

Signed-off-by: Evan Chuchen <evan_chuchen@jabil.com>