firewall: Always enable connection tracking for GRE

If this module is not being loaded, the kernel will mark any
GRE connection as INVALID in connection tracking, which will
be then silently dropped by a firewall rule.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>

config/rootfiles/core/147/filelists/files

@@ -2,3 +2,4 @@ etc/system-release
 etc/issue
 srv/web/ipfire/cgi-bin/credits.cgi
 var/ipfire/langs
+etc/rc.d/init.d/firewall