selftests: forwarding: Install no_forwarding.sh

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

Fixes: 476a4f05 ("selftests: forwarding: add a no_forwarding.sh test")
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>