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.
There's nothing wrong the current code, but by declaring kobjop_t as a
(void (*)(void)), we can avoid gcc warning -Wcast-function-type, which
has an exception for that type.
To be clear, this doesn't fix a problem for code in-tree with current
warnings. I recently ran into an out-of-tree module which uses this
warning when compiled with gcc via -Wextra.
Other solutions could be either to change the cast in the assignment in
KOBJMETHOD to first cast through that type, or just to leave it to
clients to disable (or not enable) the warning (i.e. take no change