libnetfilter_conntrack: switch to git

libnetfilter_conntrack: switch to git

In order to build conntrack-tools from git, a newer version of
libnetfilter_conntrack is required. As 1.0.6 is currently the latest
release, switch to git.

b0a7cf7 include: expose a copy of nf_conntrack_common.h
f68f7b3 conntrack: fix missing break in setobjopt_undo_dnat()
79dac5a conntrack: revert getobjopt_is_nat() condition
b266523 libnetfilter_conntrack: bump version to 1.0.7
e870432 labels: don't crash on NULL labelmap

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>