ulogd: link conditionally to libpthread

libpthread is only needed for the database backends so don't link
against it if it's not available.

Fixes:
http://autobuild.buildroot.net/results/527/527d9d4f42af0ffc110e4f52701df279f466a89a/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>