package/squid: security bump to version 4.12

- Fix CVE-2020-15049: Cache Poisoning Issue in HTTP Request processing
- Fix CVE-2020-14058: Denial of Service issue in TLS handshake
- Fix CVE-2020-14059: Denial of Service when using SMP cache

This version also fix a build failure with systemd

Fixes:
 - http://autobuild.buildroot.org/results/4f586c497577d6c96289e821430fa2c2f61eda2a



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>