selftests/net: msg_zerocopy enable build with older kernel headers

Explicitly define SO_EE_ORIGIN_ZEROCOPY.
This makes the test program build with older kernel headers,
e.g. from Debian 9.

Signed-off-by: Thomas Meyer <thomas@m3y3r.de>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>