sound: Add export.h for THIS_MODULE/EXPORT_SYMBOL where needed

These aren't modules, but they do make use of these macros, so
they will need export.h to get that definition.  Previously,
they got it via the implicit module.h inclusion.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
diff --git a/sound/synth/emux/emux_oss.c b/sound/synth/emux/emux_oss.c
index 87e4220..319754c 100644
--- a/sound/synth/emux/emux_oss.c
+++ b/sound/synth/emux/emux_oss.c
@@ -25,6 +25,7 @@
 
 #ifdef CONFIG_SND_SEQUENCER_OSS
 
+#include <linux/export.h>
 #include <asm/uaccess.h>
 #include <sound/core.h>
 #include "emux_voice.h"