gluon-mesh-batman-adv: respondd: determine compat version from release version

We cannot add the same file (here: /lib/gluon/mesh-batman-adv/compat) to
two, installed packages. Therefore, instead of determining the compat
version number from this file, infer it from the batman-adv release
version number instead.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>