HID: i2c-hid: fix build

Add a forgotten include that I've by mistake omitted when resolving
merge conflict in ead0687fe30 ("HID: i2c-hid: support regulator power
on/off").

Fixes: ead0687fe30 ("HID: i2c-hid: support regulator power on/off")
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>