Hide tunable definitions behind _KERNEL

Some userspace code include sys/kernel.h. Namely, some OpenZFS tests do
this, and it was causing breakage after r365945 due to a lack of bool
typedef. Userspace should not need the TUNABLE_** stuff, so hide it
behind an #ifdef _KERNEL.

Sorry for the breakage.

Reported by: andrew, Michael Butler, Jenkins
Discussed with: kevans, allanjude