ide: export ide_doubler

This patch fixes the following build error:

<--  snip  -->

...
  Building modules, stage 2.
  MODPOST 1204 modules
ERROR: "ide_doubler" [drivers/ide/ide-core.ko] undefined!
...
make[2]: *** [__modpost] Error 1

<--  snip  -->

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>