Fix detection of qb_log_thread_priority_set

This fixes detection of libqb function qb_log_thread_priority_set
if it was installed outside of the standard library search
path, in my case /opt.

Signed-off-by: Ruben Kerkhof <ruben@rubenkerkhof.com>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>