platform/x86: intel_mid_powerbutton needs INPUT

intel_mid_powerbtn.c uses input interfaces, so it should depend
on INPUT to fix build errors when CONFIG_INPUT is not enabled:

intel_mid_powerbtn.c:(.text+0x56ca8f): undefined reference to `input_event'
intel_mid_powerbtn.c:(.devinit.text+0x2e7b4): undefined reference to `input_allocate_device'
intel_mid_powerbtn.c:(.devinit.text+0x2e7ff): undefined reference to `input_set_capability'
intel_mid_powerbtn.c:(.devinit.text+0x2e84a): undefined reference to `input_register_device'
intel_mid_powerbtn.c:(.devinit.text+0x2e88b): undefined reference to `input_free_device'
intel_mid_powerbtn.c:(.devexit.text+0x42f0): undefined reference to `input_unregister_device'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Hong Liu <hong.liu@intel.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>