usb: storage: Fix link error

Fix allmodconfig link error introduced by commit 75b9130e
("usb: storage: Add usb_stor_dbg, reduce object size")

Export the symbol usb_stor_dbg.
Add export.h
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/storage/debug.c

@@ -43,6 +43,7 @@
  */
 
 #include <linux/cdrom.h>
+#include <linux/export.h>
 #include <scsi/scsi.h>
 #include <scsi/scsi_cmnd.h>
 #include <scsi/scsi_dbg.h>
@@ -193,3 +194,4 @@ int usb_stor_dbg(const char *fmt, ...)
 
 	return r;
 }
+EXPORT_SYMBOL_GPL(usb_stor_dbg);