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/isa/Kconfig b/sound/isa/Kconfig
index bf1ab34..ea8ecc5 100644
--- a/sound/isa/Kconfig
+++ b/sound/isa/Kconfig
@@ -389,6 +389,7 @@
 	def_tristate SND_SEQUENCER && SND_SBAWE
 	select SND_SEQ_MIDI_EMUL
 	select SND_SEQ_VIRMIDI
+	select SND_SYNTH_EMUX
 
 config SND_SB16_CSP
 	bool "Sound Blaster 16/AWE CSP support"