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.

Reviewed-on: http://gerrit.openafs.org/4095
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 2681c49ada5679922f949e9c7d0010f2ee705dab)

Change-Id: Ie1858dc4e7dfc6e9db4a4f63deda8018645f9111
Reviewed-on: http://gerrit.openafs.org/4162
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>