HID: avoid sparse warning in HID_COMPAT_LOAD_DRIVER

Impact: include a prototype for the exported function in the macro

Fix about 20 of this warnings:

  drivers/hid/hid-a4tech.c:162:1: warning: symbol 'hid_compat_a4tech' was not declared. Should it be static?

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>