mctp: Remove the repeated declaration

Function 'mctp_dev_get_rtnl' is declared twice, so remove the
repeated declaration.

Cc: Jeremy Kerr <jk@codeconstruct.com.au>
Cc: Matt Johnston <matt@codeconstruct.com.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>