Fix ENA build when integrated into kernel

Provide missing rules for ena_datapath.c and ena_netmap.c,
which prevented the ENA driver from building.
This issue was showing up only when building the driver statically
into the kernel.

PR: 248116
Submitted by: Artur Rojek <ar@semihalf.com>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D25796
Obtained from: Semihalf
Sponsored by: Amazon, Inc.