staging: exfat: Update MAINTAINERS file

Add a L: tag so get_maintainers.pl output includes the linux-fsdevel list

Signed-off-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191023055353.695275-1-Valdis.Kletnieks@vt.edu


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>