chore(Data.Complex): add @[simp] to Complex.conj_ofReal (#14648)

This came up in #14601: it seems this `simp` lemma is missing, and as a consequence the `simp` set wouldn't converge.

Zulip discussion: https://leanprover.zulipchat.com/#narrow/stream/287929-mathlib4/topic/Should.20docs.23Complex.2Econj_ofReal.20be.20.60.40.5Bsimp.5D.60.3F

Author: Vierkantor

Mathlib/Data/Complex/Basic.lean

@@ -560,6 +560,7 @@ theorem conj_im (z : ℂ) : (conj z).im = -z.im :=
   rfl
 #align complex.conj_im Complex.conj_im
 
+@[simp]
 theorem conj_ofReal (r : ℝ) : conj (r : ℂ) = r :=
   ext_iff.2 <| by simp [star]
 #align complex.conj_of_real Complex.conj_ofReal