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>