iser: Add kernel build glue

'device iser' is documented in iser(4) but not supported. Hook it up
to the build.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44687

(cherry picked from commit 60bb979b3c3224f01e96f7e3c92a270977d2587b)