PM: Export device_pm_schedule_removal

Move the declaration of device_pm_schedule_removal() to device.h
and make it exported, as it will be used directly by some drivers
for unregistering device objects during suspend/resume cycles in a
safe way.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>