btrfs-progs: give raid6.c its exported prototypes

btrfs-progs: give raid6.c its exported prototypes

raid6.c is built without access to the prototypes of functions it
exports.

  warning: symbol 'raid6_gen_syndrome' was not declared.  Should it be static?

They could be changed and get out of sync of the exported prototypes
without errors.   So we add disk-io.h, and its dependency ctree.h, so
that it has a chance to check that its exported prototypes are correct.

Signed-off-by: Zach Brown <zab@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>