drivers/rapidio/rio-driver.c: fix missing include of <linux/rio_drv.h>

Include <linux/rio_drv.h> for the missing declarations of functions
exported from this file.  Fixes the following sparse warnings:

  drivers/rapidio/rio-driver.c:53:16: warning: symbol 'rio_dev_get' was not declared. Should it be static?
  drivers/rapidio/rio-driver.c:70:6: warning: symbol 'rio_dev_put' was not declared. Should it be static?
  drivers/rapidio/rio-driver.c:150:5: warning: symbol 'rio_register_driver' was not declared. Should it be static?
  drivers/rapidio/rio-driver.c:169:6: warning: symbol 'rio_unregister_driver' was not declared. Should it be static?

Link: http://lkml.kernel.org/r/20191017114923.10888-1-ben.dooks@codethink.co.uk
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Alexandre Bounine <alex.bou9@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>