clean: use f(void) instead of f() to declare a pointer to a function without arguments

Explicitly state that menu_item functions like clean_cmd don't take
any arguments by using void instead of an empty parameter list.

Found using gcc -Wstrict-prototypes.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'rs/clean-menu-item-defn' into maint

* rs/clean-menu-item-defn:
  clean: use f(void) instead of f() to declare a pointer to a function without arguments