selftests: forwarding: fix missing backslash

Fix missing backslash, introduced in f62c5acc.  Causes all tests to
not be installed.

Fixes: f62c5acc ("selftests/net/forwarding: add missing tests to Makefile")
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
Acked-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20220518151630.2747773-1-troglobit@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>