Fix kernel buuild with (CONFIG_COMPAT && ! CONFIG_BLOCK)

Commit 02a5e0ac ("BLOCK: Hide the
contents of linux/bio.h if CONFIG_BLOCK=n") broke the kernel build for
the CONFIG_COMPAT && !CONFIG_BLOCK case:

    CC      fs/compat_ioctl.o
  In file included from include/linux/raid/md_k.h:19,
                   from include/linux/raid/md.h:54,
                   from fs/compat_ioctl.c:25:
  include/linux/raid/../../../drivers/md/dm-bio-list.h: In bio_list_:
  include/linux/raid/../../../drivers/md/dm-bio-list.h:40: error: dereferencing pointer to incomplete type
  include/linux/raid/../../../drivers/md/dm-bio-list.h: In bio_list_:
  include/linux/raid/../../../drivers/md/dm-bio-list.h:48: error: dereferencing pointer to incomplete type
  include/linux/raid/../../../drivers/md/dm-bio-list.h:51: error: dereferencing pointer to incomplete type
  include/linux/raid/../../../drivers/md/dm-bio-list.h: In bio_list_:
  include/linux/raid/../../../drivers/md/dm-bio-list.h:64: error: deref...