LWP: remove ucontext header from preempt module

LWP: remove ucontext header from preempt module

it's already included by header. don't include it again.

Reviewed-on: http://gerrit.openafs.org/4076
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit fbac809a46376fcf60d3dcedc0be962c43d9c74e)

Change-Id: I8f56d5cc90957285cfd765f791b584eb36062070
Reviewed-on: http://gerrit.openafs.org/4082
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>