openntpd: needs NPTL

Openntpd in fact needs nptl threads, basic threads aren't enough. Fixes:
http://autobuild.buildroot.net/results/f4d/f4dd964231a5dbbc12a7237127fa9c45d6aa0d06/



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