chore(data/int/basic): mark cast_sub with simp attribute (#2687)

I think the reason this didn't have the `simp` attribute already was from the time when `sub_eq_add_neg` was a `simp` lemma, so it wasn't necessary. I'm adding the `simp` attribute back now that `sub_eq_add_neg` is not a `simp` lemma.

Author: ChrisHughes24

src/data/int/basic.lean

@@ -1164,7 +1164,7 @@ end
 | (n : ℕ) := cast_neg_of_nat _
 | -[1+ n] := (neg_neg _).symm
 
-@[norm_cast] theorem cast_sub [add_group α] [has_one α] (m n) : ((m - n : ℤ) : α) = m - n :=
+@[simp, norm_cast] theorem cast_sub [add_group α] [has_one α] (m n) : ((m - n : ℤ) : α) = m - n :=
 by simp [sub_eq_add_neg]
 
 @[simp] theorem cast_eq_zero [add_group α] [has_one α] [char_zero α] {n : ℤ} : (n : α) = 0 ↔ n = 0 :=