linux define ucontext properly

linux define ucontext properly

ucontext requires glibc version info to engage.
glibc info comes from features.h, which if we include afs_sysnames.h,
we get... so, let's swap these.

Reviewed-on: http://gerrit.openafs.org/2868
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 364691145698c8f9b87466cd9d9e2c54f8c6aaa9)
Change-Id: Id9b143811a988095cda9d8f09b67732e3d18156b
Reviewed-on: http://gerrit.openafs.org/2935