OpenBSD: curproc has moved in OpenBSD 4.8

OpenBSD: curproc has moved in OpenBSD 4.8

With OpenBSD 4.8, curproc has moved from h/proc.h to
h/systm.h. Fix rx_event.c to reflect this change.

Reviewed-on: http://gerrit.openafs.org/3749
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit e4660dd8cfb314971224dac5d1d13426d56494dd)

Change-Id: Ie59100172daae48dea985749e07416daea88b8d2
Reviewed-on: http://gerrit.openafs.org/3923
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>