Revert "LWP: remove ucontext header from preempt module"

Revert "LWP: remove ucontext header from preempt module"

This reverts commit fbac809a46376fcf60d3dcedc0be962c43d9c74e.

The ucontext header is only included in lwp.h for platforms that use
ucontext as a means for context-switching in LWP. The preempt module
needs the ucontext header for signal handling on some platforms (at
least Solaris). So, ucontext.h may not be included by lwp.h on these
platforms, if that platform does not use ucontext for LWP
context-switching. So we need to explicitly include ucontext.h in
preempt.c.

Change-Id: Ie08f20c7375d6ff1b185f64da70c983dbe19ac47
Reviewed-on: http://gerrit.openafs.org/4095
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>