HID: logitech-dj: merge header file into the source

There is no point in keeping the header in a separate file, nobody
but hid-logitech-dj should have access to its content.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Andrew de los Reyes <adlr@chromium.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>