netlink: use (void) for function definitions with no arguments

For some of these Clang produced a warning that "a function declaration
without a prototype is deprecated in all versions of C". In other cases
the function defintion used () which did not match the header
declaration, which used (void).

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 43d0c2ddd2556841717a50925f713c9c70d3936e)