[ALSA] Make ak4531 local to ens1370 driver

The ak4531 module is used only by ens1370 driver (and unlikely that
any other will use it ever).  Let's make it local to ens1370.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/pci/Makefile b/sound/pci/Makefile
index 85ef14b..65b25d2 100644
--- a/sound/pci/Makefile
+++ b/sound/pci/Makefile
@@ -13,7 +13,7 @@
 snd-cmipci-objs := cmipci.o
 snd-cs4281-objs := cs4281.o
 snd-cs5530-objs := cs5530.o
-snd-ens1370-objs := ens1370.o
+snd-ens1370-objs := ens1370.o ak4531_codec.o
 snd-ens1371-objs := ens1371.o
 snd-es1938-objs := es1938.o
 snd-es1968-objs := es1968.o