obexd: fix QA warnings by creating empty package obex-plugins

fix the following warnings:
WARNING: For recipe obexd, the following files/directories were installed but not shipped in any package:
WARNING:   /usr/lib
WARNING:   /usr/lib/obex
WARNING:   /usr/lib/obex/plugins

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>