ALSA: emux: Fix/cleanup old ifdef CONFIG_PROC_FS

Build emux_proc.o and drop the unneeded ifdefs.
Replace the left CONFIG_PROC with the new CONFIG_SND_PROC_FS.

Along with this, fix the build of emux_oss.o in Makefile, too.

Signed-off-by: Takashi Iwai <tiwai@suse.de>