ice(4): Add ice_rdma.c to module Makefile

When ice is built as a module, it can't be loaded due to unresolved
symbol. Ensuring in Makefile that the ice_rdma.c is built fixes the
problem.

Signed-off-by: Bartosz Sobczak <bartosz.sobczak@intel.com>
Signed-off-by: Eric Joyner <erj@freebsd.org>

Reviewed by: erj@
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D35687