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>

sound/isa/gus/gus_volume.c

@@ -19,6 +19,7 @@
  */
 
 #include <linux/time.h>
+#include <linux/export.h>
 #include <sound/core.h>
 #include <sound/gus.h>
 #define __GUS_TABLES_ALLOC__