pata_of_platform: fix compile error

I failed to include <linux/libata.h>, causing this error:

  drivers/ata/pata_of_platform.c:93: error: 'ata_platform_remove_one' undeclared here (not in a function)

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>