raid6: remove the <linux/export.h> include from recov.c

There is no exported symbol left in recov.c, so the include is now
unnecessary, and breaks the raid6test build. Remove it.

Signed-off-by: WANG Xuerui <git@xen0n.name>
Link: https://lore.kernel.org/r/20230731104911.411964-2-kernel@xen0n.name
Signed-off-by: Song Liu <song@kernel.org>