libvncserver: OpenSSL support needs NPTL thread support

The OpenSSL support of libvncserver already has a dependency on thread
support, but it in fact requires NPTL threading.

Fixes:

  http://autobuild.buildroot.org/results/be2/be20dec9ffe7de3adc46a834852b20353d39baff/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>