package/mono: Fixing wrong MonoPosixHelper location

This patch solves an host path leaking in /etc/mono/config
that leads to searching libmonoposixhelper into the wrong path.
This bug is fixed upstream, but not included in the current stable
release.

Fixes bug #8651.

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>