selftests: forwarding: Install local_termination.sh

When using the Makefile from tools/testing/selftests/net/forwarding/
all tests should be installed. Add local_termination.sh to the list of
"to be installed tests" where it has been missing so far.

Fixes: 90b9566a ("selftests: forwarding: add a test for local_termination.sh")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>