ulogd: disable DB backends if no threads

Database backends need threads even if sqlite does not.
The mysql client libs need them too but include them in the exclusion
closure anyway to state it very clearly. Fixes:
http://autobuild.buildroot.net/results/1ce/1cece0db309dfe5cecc69e88b02428615ed87316/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>