comment the prove of kronecker_bigop_scal_in_l to compile

FEM/kronecker.v

@@ -139,11 +139,11 @@ Proof.
 intros n j a.
 induction n.
 (* *)
-erewrite <- bigop_plus_0.
+(*erewrite <- bigop_plus_0.
 f_equal.
 admit.
 apply functional_extensionality; intros i; f_equal.
- 
+ *)
 (*
 rewrite bigop_plus_0; easy.
 (* *)