LINUX: Fix includes for fatal_signal_pending test

LINUX: Fix includes for fatal_signal_pending test

Commit 8b6ae289 (LINUX: Avoid lookup ENOENT on fatal signals) added a
configure test for fatal_signal_pending(). However, this check fails
incorrectly ever since Linux 4.11, because fatal_signal_pending() was moved
from linux/sched.h to linux/sched/signal.h in Linux commit 2a1f062a
(sched/headers: Move signal wakeup [...]). Fix this by including
linux/sched/signal.h if we have it during the configure test.

A false negative on this configure test doesn't break the build, but
it disables one of our safeguards preventing incorrect negative
dentries at runtime. The function fatal_signal_pending() hasn't
changed in quite some time (except for what header it lives in); it
was introduced in Linux 2.6.25 via Linux commit f776d12d (Add
fatal_signal_pending). So to try to avoid this mistake again in the
future, make it so a missing fatal_signal_pending() breaks the build
if we're on Linux 2.6.25+.

Change-Id: Id0b91b2f24e2ea87c9c900076ab7ab1fcab3d304
Reviewed-on: https://gerrit.openafs.org/14508
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>