selftests: net: add in_netns.sh to TEST_PROGS

Script in_netns.sh isn't installed.
--------------------
running psock_fanout test
--------------------
./run_afpackettests: line 12: ./in_netns.sh: No such file or directory
[FAIL]
--------------------
running psock_tpacket test
--------------------
./run_afpackettests: line 22: ./in_netns.sh: No such file or directory
[FAIL]

In current code added in_netns.sh to be installed.

Fixes: cc30c93f

 ("selftests/net: ignore background traffic in psock_fanout")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>

Signed-off-by: David S. Miller <davem@davemloft.net>