package/squid: add SELinux module

Support for squid is added by the services/squid module in the SELinux
refpolicy.

Moreover, add apache module as for now it is unconditionally used in
squid.fc

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>