package/x11vnc: fix build issue with --without-ssl

Fixes:
http://autobuild.buildroot.net/results/873/873ed8f2ade1d969abdff15b7b6d63e04819af9a



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>