ALSA: synth: Select snd-emux-synth explicitly

Instead of the non-standard way to enable the build of snd-emux-synth
module inside Makefile, rewrite Kconfig to select the item explicitly
from each driver (sbawe and emu10k1).  This is the standard way.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/Kconfig b/sound/Kconfig
index d2fadbe..d7d2aac 100644
--- a/sound/Kconfig
+++ b/sound/Kconfig
@@ -110,6 +110,8 @@
 
 source "sound/x86/Kconfig"
 
+source "sound/synth/Kconfig"
+
 endif # SND
 
 menuconfig SOUND_PRIME