net-im/libsignal-client: Revert update to 0.9.7

This version of net-im/libsignal-client is not compatible with
net-im/signal-cli 0.9.0. Roll back. A roll-back of net-im/signal-cli
will follow.

This reverts commit d3d2f99693aa0cebdd70795b7baadea9b0601dcf.