selftests: forwarding: Remove executable bits from lib.sh

The lib.sh script is meant to be sourced from other scripts, not executed
directly. Therefore, remove the executable bits from lib.sh's permissions.

Fixes: fe32dffd ("selftests: forwarding: add TCPDUMP_EXTRA_FLAGS to lib.sh")
Tested-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>