chore(Algebra/Group/Subgroup/Basic): tag instIsMulTorsionFree with to_additive (leanprover-community#34536)

Free `to_additive` that is missing for some reason.

Co-authored-by: tb65536 <thomas.l.browning@gmail.com>

Author: tb65536

Mathlib/Algebra/Group/Subgroup/Basic.lean

@@ -216,6 +216,7 @@ theorem pi_eq_bot_iff (H : ∀ i, Subgroup (f i)) : pi Set.univ H = ⊥ ↔ ∀
 
 end Pi
 
+@[to_additive]
 instance instIsMulTorsionFree [IsMulTorsionFree G] : IsMulTorsionFree H where
   pow_left_injective n hn a b := by
     have := pow_left_injective hn (M := G) (a₁ := a) (a₂ := b)