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

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: 7e9838b7

 ("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>