Fix rnd_boss_restrict=true will trigger a bug.

Let `rnd_boss_restrict` use `util.getval` interface to make it support defination like `rnd_boss_restrict = true` (in `data/talents/celestial/combat.lua:130`)

Bug report:
```
Lua Error: /mod/class/GameState.lua:2218: attempt to call field 'rnd_boss_restrict' (a boolean value)
```

See merge request !639