block: make sure that REQ_* types are seen even with CONFIG_BLOCK=n

These form the basis of the basic WRITE etc primitives, so we
need them to be always visible. Otherwise we see errors like:

	mm/filemap.c:2164: error: 'REQ_WRITE' undeclared
	fs/read_write.c:362: error: 'REQ_WRITE' undeclared
	fs/splice.c:1108: error: 'REQ_WRITE' undeclared
	fs/aio.c:1496: error: 'REQ_WRITE' undeclared

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>