ansible/osmocom-jenkins-slave: add libnftables-dev

Install libnftables-dev, so osmo-upf can build successfully in the
coverity jenkins job.

Change-Id: Id72ce8ed5b662085a16f56756f8f93627834d2e2

ansible/roles/osmocom-jenkins-slave/tasks/osmocom_build_deps.yml

@@ -66,6 +66,7 @@
       - libgnutls28-dev
       - libgps-dev
       - libgsm1-dev
+      - libnftables-dev
       - libnl-3-dev
       - libnl-route-3-dev
       - libmnl-dev