vty: fix osmo_cpu_sched_vty_init() prototype
Change return type of osmo_cpu_sched_vty_init() from void to int
to match its implementation.
This fixes a prototype mismatch rejected by stricter compilers
(e.g. Emscripten).
Change-Id: Ia86e8063178ad82de098c2ff693cfbcd7e9f2192
