macintosh/ams: linux/platform_device.h is needed

ams.h uses struct platform_device, so the header should be used
to prevent build errors:

drivers/macintosh/ams/ams-input.c: In function 'ams_input_enable':
drivers/macintosh/ams/ams-input.c:68:45: error: invalid use of undefined type 'struct platform_device'
   68 |         input->dev.parent = &ams_info.of_dev->dev;
drivers/macintosh/ams/ams-input.c: In function 'ams_input_init':
drivers/macintosh/ams/ams-input.c:146:51: error: invalid use of undefined type 'struct platform_device'
  146 |         return device_create_file(&ams_info.of_dev->dev, &dev_attr_joystick);
drivers/macintosh/ams/ams-input.c: In function 'ams_input_exit':
drivers/macintosh/ams/ams-input.c:151:44: error: invalid use of undefined type 'struct platform_device'
  151 |         device_remove_file(&ams_info.of_dev->dev, &dev_attr_joystick);
drivers/macintosh/ams/ams-input.c: In function 'ams_input_init':
drivers/macintosh/ams/ams-input.c:147:1: error: control reaches end of non-void function [-Werror=return-type]
  147 | }

Fixes: 233d687d

 ("macintosh: Explicitly include correct DT includes")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230829225837.15520-1-rdunlap@infradead.org