selftests/net/forwarding: add Makefile to install tests

Add missing Makefile for net/forwarding tests and include it to
the targets list, otherwise forwarding tests are not installed
in case of cross-compilation.

Signed-off-by: Vadym Kochan <vadym.kochan@plvision.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>