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: 7d157501 ("selftests/net: GRO coalesce test")
Fixes: 9af771d2

 ("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>