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: fe32dff ("selftests: forwarding: add TCPDUMP_EXTRA_FLAGS to lib.sh")
Tested-by: Hangbin Liu <[email protected]>
Reviewed-by: Hangbin Liu <[email protected]>
Reviewed-by: Vladimir Oltean <[email protected]>
Signed-off-by: Benjamin Poirier <[email protected]>
Reviewed-by: Przemek Kitszel <[email protected]>
Signed-off-by: Paolo Abeni <[email protected]>