xl2tpd: dump 1.3.1 in favour of git version

When the yocto kernel updated to 3.8.x, it exposed some breakage in the
xl2tpd 1.3.1 release that has been since fixed in tree.  Rather than
cherry pick patches, we'll migrate to the current HEAD.

Signed-off-by: Joe MacDonald <joe.macdonald@windriver.com>