netmap: set IFCAP_NETMAP in if_capabilities

Revision r307394 removed (by mistake) the code that sets IFCAP_NETMAP
in if_capabilities on netmap_attach. This patch reverts this change.

Reviewed by: np
Approved by: gnn (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D17987