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

[crgeddes]

https://github.com/openbmc/libmctp/blob/ba6727e6d61d19720d4ca8756f465375adc82536/core.c#L610-L611

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.