Add PF_MCTP, AF_MCTP from Linux 5.15 to bits/socket.h

[Florian Weimer]

* Joseph Myers:

> Linux 5.15 adds a new address / protocol family PF_MCTP / AF_MCTP; add
> these constants to bits/socket.h.

Double-checked against the kernel tree, looks good.  Thanks.

Florian