kselftests/net: add missed icmp.sh test to Makefile

kselftests/net: add missed icmp.sh test to Makefile

[ Upstream commit ca3676f94b8f40f52d285f9aef36dfd6725bfc14 ]

When generating the selftests to another folder, the icmp.sh test will
miss as it is not in Makefile, e.g.

  make -C tools/testing/selftests/ install \
      TARGETS="net" INSTALL_PATH=/tmp/kselftests

Fixes: 7e9838b7915e ("selftests/net: Add icmp.sh for testing ICMP dummy address responses")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>