shared/io-ell: Fix compiler error with GCC 8.0

This fixes the following error:

src/shared/io-ell.c: In function ‘io_set_disconnect_handler’:
src/shared/io-ell.c:117:7: error: cast between incompatible function types from ‘io_callback_func_t’ {aka ‘_Bool (*)(struct io *, void *)’} to ‘void (*)(struct l_io *, void *)’ [-Werror=cast-function-type]
       (l_io_disconnect_cb_t) callback,
       ^