kselftests/net: add missed setup_loopback.sh/setup_veth.sh to Makefile

When generating the selftests to another folder, the include file
setup_loopback.sh/setup_veth.sh for gro.sh/gre_gro.sh are missing as
they are not in Makefile, e.g.

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

Fixes: 7d15750 ("selftests/net: GRO coalesce test")
Fixes: 9af771d ("selftests/net: allow GRO coalesce test on veth")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/Makefile

@@ -30,7 +30,7 @@ TEST_PROGS += ioam6.sh
 TEST_PROGS += gro.sh
 TEST_PROGS += gre_gso.sh
 TEST_PROGS += cmsg_so_mark.sh
-TEST_PROGS_EXTENDED := in_netns.sh
+TEST_PROGS_EXTENDED := in_netns.sh setup_loopback.sh setup_veth.sh
 TEST_GEN_FILES =  socket nettest
 TEST_GEN_FILES += psock_fanout psock_tpacket msg_zerocopy reuseport_addr_any
 TEST_GEN_FILES += tcp_mmap tcp_inq psock_snd txring_overwrite