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.

MFC after: 2 weeks
Reported by: clang
Tested with: clang 3.8.1, gcc 4.2.1, gcc 5.3.0
Sponsored by: EMC / Isilon Storage Division