freeradius3: add missing conffiles to Makefile

freeradius3: add missing conffiles to Makefile

Config files
/etc/freeradius3/policy.d/accounting
/etc/freeradius3/policy.d/filter
/etc/freeradius3/proxy.conf
/etc/freeradius3/sites-available/default
and link
/etc/freeradius3/sites-enabled/default
are in the freeradius3 package and are mentioned in the main config file
/etc/freeradius3/radiusd.conf
Thus, they must be explicitly specified in the Makefile.

File
/etc/freeradius3/sites/default
is not included in the package, is not created during installation,
is not mentioned in the main config file and should therefore be excluded
from the Makefile.

(backported from commit f6974b8)
Signed-off-by: Alexey Dobrovolsky <dobrovolskiy.alexey@gmail.com>