ASoC: rt5645: fix implicit declaration error

kbuild robot reports a implicit declaration of function
'rt5645_irq_detection' error.

Signed-off-by: Bard Liao <bardliao@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/sound/soc/codecs/rt5645.c b/sound/soc/codecs/rt5645.c
index 16de9ba..346ac45 100644
--- a/sound/soc/codecs/rt5645.c
+++ b/sound/soc/codecs/rt5645.c
@@ -2535,6 +2535,8 @@
 	return rt5645->jack_type;
 }
 
+static int rt5645_irq_detection(struct rt5645_priv *rt5645);
+
 int rt5645_set_jack_detect(struct snd_soc_codec *codec,
 	struct snd_soc_jack *hp_jack, struct snd_soc_jack *mic_jack,
 	struct snd_soc_jack *btn_jack)