mtd: sh_flctl: enable driver compilation with COMPILE_TEST

This helps increasing build testing coverage.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>