ALSA: ctxfi - Add depends on X86

The ctxfi driver requires explicitly the 4k page size, and gives a
build error on architectures with non-4k pages.
As a workaround, just add the kconfig dependency on X86, which is
the only architecture ever tested.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/Kconfig

@@ -277,6 +277,7 @@ config SND_CS5535AUDIO
 
 config SND_CTXFI
 	tristate "Creative Sound Blaster X-Fi"
+	depends on X86
 	select SND_PCM
 	help
 	  If you want to use soundcards based on Creative Sound Blastr X-Fi