selftests: forwarding: add shebang for sch_red.sh

selftests: forwarding: add shebang for sch_red.sh

RHEL/Fedora RPM build checks are stricter, and complain when executable
files don't have a shebang line, e.g.

*** WARNING: ./kselftests/net/forwarding/sch_red.sh is executable but has no shebang, removing executable bit

Fix it by adding shebang line.

Fixes: 6cf0291f9517 ("selftests: forwarding: Add a RED test for SW datapath")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20220922024453.437757-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>