usnic: check for <infiniband/verbs.h>

The usnic provider uses <infiniband/verbs.h> to get some of the verbs
kernel ABI enums, so check for it in the usnic configury.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Merge pull request #204 from jsquyres/pr/usnic-infiniband-verbsh-check

usnic: check for <infiniband/verbs.h>