uuid: don't export guid_index and uuid_index

These are only used in uuid.c and vsprintf.c and aren't something modules
should use directly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>