selftests: net: add udpgso* to TEST_GEN_FILES

The generated files udpgso* shouldn't be part of TEST_PROGS, they are
used by udpgso.sh and udpgsp_bench.sh. They should be added to the
TEST_GEN_FILES to get installed without being added to the main
run_kselftest.sh script.

Fixes: 3a687bef ("selftests: udp gso benchmark")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>