squid: drop session external acl helper, use file_userip

The session external acl helper needs berkeleydb, switch to the basic
file_userip one which was my original intention (renamed from ip_user in
the 3.3.x series). Fixes:
http://autobuild.buildroot.net/results/635/6359137cde3b293d46df732836b8966d30f23cbc/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>