msg param in mctp_message_tx should be const ptr and ptr to const

[crgeddes]

libmctp/core.c

Lines 610 to 611 in ba6727e

	int mctp_message_tx(struct mctp *mctp, mctp_eid_t eid,
	void *msg, size_t msg_len)

I appears that the value stored at the pointer and the value of the pointer itself are never altered so they should be labeled as const.