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>

tools/testing/selftests/net/Makefile

@@ -12,7 +12,7 @@ TEST_PROGS += udpgro_bench.sh udpgro.sh test_vxlan_under_vrf.sh reuseport_addr_a
 TEST_PROGS += test_vxlan_fdb_changelink.sh so_txtime.sh ipv6_flowlabel.sh
 TEST_PROGS += tcp_fastopen_backup_key.sh fcnal-test.sh l2tp.sh traceroute.sh
 TEST_PROGS += fin_ack_lat.sh fib_nexthop_multiprefix.sh fib_nexthops.sh
-TEST_PROGS += altnames.sh icmp_redirect.sh ip6_gre_headroom.sh
+TEST_PROGS += altnames.sh icmp.sh icmp_redirect.sh ip6_gre_headroom.sh
 TEST_PROGS += route_localnet.sh
 TEST_PROGS += reuseaddr_ports_exhausted.sh
 TEST_PROGS += txtimestamp.sh