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)

mentioned in commit 50974198

Status changed to merged