staging: exfat: Update MAINTAINERS file

Add a L: tag so get_maintainers.pl output includes the linux-fsdevel list
Signed-off-by: NValdis Kletnieks <Valdis.Kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191023055353.695275-1-Valdis.Kletnieks@vt.eduSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -6159,6 +6159,7 @@ F:	include/uapi/linux/mii.h
 
 EXFAT FILE SYSTEM
 M:	Valdis Kletnieks <valdis.kletnieks@vt.edu>
+L:	linux-fsdevel@vger.kernel.org
 S:	Maintained
 F:	drivers/staging/exfat/