tests/boot-stage1: Fix build of kcanary module

In bfe9c928

 the default kernel has been
updated to version 4.14 and the declarations for allow_signal() and
signal_pending() are no longer exposed via kthread.h, so let's actually
use the right header files.

I've added a condition for kernel 4.10 and upwards to include the
linux/sched/signal.h header file, because that got introduced in version
4.10. Even if the declaration would still reside in kthread.h (I haven't
checked) for version 4.10 it won't hurt and the compilation will still
succeed.

Tested against kernel 4.9 and 4.14 and the build now succeeds.

Signed-off-by: aszlig <aszlig@nix.build>