kobj: avoid gcc -Wcast-function-type

The actual type of kobjop_t is arbitrary, it is only used as a generic
function pointer type. Declare it as void (*)(void) in order to avoid
gcc's -Wcast-function-type, which is included in -Wextra.

Reviewed by: avg, jhb
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D28769