fix: add default_instance attribute to instHSMul and instHVAdd (#1501)

Fixes issue in https://leanprover.zulipchat.com/#narrow/stream/287929-mathlib4/topic/Heterogeneous.20scalar.20multiplication

Mathlib/Algebra/Group/Defs.lean

@@ -79,21 +79,21 @@ class SMul (M : Type _) (α : Type _) where
   smul : M → α → α
 #align has_smul SMul
 
-instance instHVAdd [VAdd α β] : HVAdd α β β where
-  hVAdd := VAdd.vadd
-
-instance instHSMul [SMul α β] : HSMul α β β where
-  hSMul := SMul.smul
-
 infixl:65 " +ᵥ " => HVAdd.hVAdd
 infixl:65 " -ᵥ " => VSub.vsub
 infixr:73 " • " => HSMul.hSMul
 
-attribute [to_additive] Mul Div HMul instHMul HDiv instHDiv instHSMul HSMul
-attribute [to_additive (reorder := 1)] Pow instHPow HPow
+attribute [to_additive] Mul Div HMul instHMul HDiv instHDiv HSMul
+attribute [to_additive (reorder := 1)] Pow HPow
 attribute [to_additive (reorder := 1 5)] HPow.hPow
 attribute [to_additive (reorder := 1 4)] Pow.pow
 
+@[to_additive (attr := default_instance)]
+instance instHSMul [SMul α β] : HSMul α β β where
+  hSMul := SMul.smul
+
+attribute [to_additive (reorder := 1)] instHPow
+
 universe u
 
 variable {G : Type _}