package/dovecot-pigeonhole: fix build with per-package directories

Fix wrong path in usr/lib/dovecot-config which was copied from the
dovecot staging dir.

Fixes:
http://autobuild.buildroot.net/results/5fb/5fb1cd57bc3fdf4f75019c7b25d65ef887eea539/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>