znc: add mandatory dependency on threads

Threads support is mandatory since version 1.7.0:
https://github.com/znc/znc/commit/582cd635f5a047761b1052a40088ab86e62fffc1

Fixes:
 - http://autobuild.buildroot.net/results/cc56c7dbee594056288296398772ef9e22846299



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>