init: INITRAMFS_PRESERVE_MTIME should depend on BLK_DEV_INITRD

init: INITRAMFS_PRESERVE_MTIME should depend on BLK_DEV_INITRD

INITRAMFS_PRESERVE_MTIME is only used in init/initramfs.c and
init/initramfs_test.c.  Hence add a dependency on BLK_DEV_INITRD, to
prevent asking the user about this feature when configuring a kernel
without initramfs support.

Fixes: 1274aea127b2e8c9 ("initramfs: add INITRAMFS_PRESERVE_MTIME Kconfig option")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>