freeswitch-stable: add FreeTDM support

The OpenWrt telephony feed already has DAHDI support. This commit
enables FreeTDM so FreeSWITCH can make use of DAHDI via libpri.

This does not change the existing FreeSWITCH packages as FreeTDM is a
separate library in the FreeSWITCH distribution, hence no revision bump.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>