x11vnc: fix build without ipv6 support

Fixes http://autobuild.buildroot.net/results/1119dbf055edf281451a9e4216fdebc0b07fffae/



configure script uses --without-ipv6 instead of --disable-ipv6, and configure
option is only used for libraries, not the x11vnc application.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>