HID: fix I2C_HID not selected when I2C_HID_OF_ELAN is

When I2C_HID_OF_ELAN is set, we need to turn on I2C_HID_CORE to
ensure we get all the HID requirements.

Fixes: bd3cba00

 ("HID: i2c-hid: elan: Add support for Elan eKTH6915 i2c-hid touchscreens")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>