module: Update prototype for ref_module (formerly use_module)

Commit 9bea7f23 renamed use_module to
ref_module (and changed its return value), but forgot to update this
prototype in module.h.

Signed-off-by: Anders Kaseorg <andersk@ksplice.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>