selftests: net: use TEST_PROGS_EXTENDED

When a script file that isn't generated uses the variable
TEST_GEN_PROGS_EXTENDED and a 'make -C tools/testing/selftests clean' is
performed the script file gets removed and git shows the file as
deleted. For script files that isn't generated TEST_PROGS_EXTENDED
should be used.

Fixes: 9faedd64

 ("selftests: net: add in_netns.sh TEST_GEN_PROGS_EXTENDED")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>