squid: fix kerberos build failure

The configure script tries to autodetect kerberos 5 support and seems to
fail under some circumnstances.
Just disable it since we don't have kerberos support in buildroot.
Fixes:
http://autobuild.buildroot.net/results/c9e590a1034c3869cb7391984ce8281ec4c3ef9c/build-end.log



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>