gnu: gajim-omemo: Update to 2.7.9.

* gnu/packages/messaging.scm (gajim-omemo) [version]: Update to
2.7.9.
[propagated-inputs]: Add python-axolotl-curve25519, python-cryptography
and python-qrcode.

Signed-off-by: Julien Lepiller <julien@lepiller.eu>