bfq: fix compile error if CONFIG_CGROUPS=n

If we don't have CGROUPS enabled, the compile ends in the
following misery:

In file included from ../block/bfq-iosched.c:105:0:
../block/bfq-iosched.h:819:22: error: array type has incomplete element type
 extern struct cftype bfq_blkcg_legacy_files[];
                      ^
../block/bfq-iosched.h:820:22: error: array type has incomplete element type
 extern struct cftype bfq_blkg_files[];
                      ^

Move the declarations under the right ifdef.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jens Axboe <axboe@fb.com>