MFstable/11 r305271:

MFC r303576:

Conditionalize code which defines sysctls per _KERNEL #ifdef guard

This resolves several issues when compiling libzpool (userspace library), i.e.
-Wimplicit-function-declaration and -Wmissing-declarations issues.

Tested with: clang 3.8.1, gcc 4.2.1, gcc 5.3.0