block: fix missing export of blk_types.h

Stephen reports:

  After merging the block tree, today's linux-next build (x86_64
  allmodconfig) failed like this:

  usr/include/linux/fs.h:11: included file 'linux/blk_types.h' is not exported

  Caused by commit 9d3dbbcd9a84518ff5e32ffe671d06a48cf84fd9 ("bio, fs:
  separate out bio_types.h and define READ/WRITE constants in terms of
  BIO_RW_* flags").

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>