chore(data/zmod): make zmod.int_cast_zmod_cast @[norm_cast] (#15753)

I was suprised that `norm_cast` couldn't do anything with an expression including `(((r : zmod 4) : ℤ) : zmod 4)`, turns out this lemma was missing a `norm_cast` attribute.

Author: Vierkantor

src/data/zmod/basic.lean

@@ -138,7 +138,7 @@ nat_cast_right_inverse.surjective
 
 /-- So-named because the outer coercion is `int.cast` into `zmod`. For `int.cast` into an arbitrary
 ring, see `zmod.int_cast_cast`. -/
-lemma int_cast_zmod_cast (a : zmod n) : ((a : ℤ) : zmod n) = a :=
+@[norm_cast] lemma int_cast_zmod_cast (a : zmod n) : ((a : ℤ) : zmod n) = a :=
 begin
   cases n,
   { rw [int.cast_id a, int.cast_id a], },