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/drivers/opl4/opl4_proc.c b/sound/drivers/opl4/opl4_proc.c
index df850b8..9b824bf 100644
--- a/sound/drivers/opl4/opl4_proc.c
+++ b/sound/drivers/opl4/opl4_proc.c
@@ -19,6 +19,7 @@
 
 #include "opl4_local.h"
 #include <linux/vmalloc.h>
+#include <linux/export.h>
 #include <sound/info.h>
 
 #ifdef CONFIG_PROC_FS