[ALSA] fix build failure due to snd-aoa

When snd-aoa is not built or built as modules, but CONFIG_SND is yes,
kernel build fails due to a bug I introduced when adding snd-aoa. This
patch fixes it.
From: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>

sound/Makefile

@@ -4,7 +4,8 @@
 obj-$(CONFIG_SOUND) += soundcore.o
 obj-$(CONFIG_SOUND_PRIME) += oss/
 obj-$(CONFIG_DMASOUND) += oss/
-obj-$(CONFIG_SND) += core/ i2c/ drivers/ isa/ pci/ ppc/ arm/ synth/ usb/ sparc/ parisc/ pcmcia/ mips/ aoa/
+obj-$(CONFIG_SND) += core/ i2c/ drivers/ isa/ pci/ ppc/ arm/ synth/ usb/ sparc/ parisc/ pcmcia/ mips/
+obj-$(CONFIG_SND_AOA) += aoa/
 
 ifeq ($(CONFIG_SND),y)
   obj-y += last.o