linux-fslc/defconfig: Add CONFIG_NET_NS=y

linux-fslc/defconfig: Add CONFIG_NET_NS=y

systemd in default oe-core configuration complains:

| Sep 11 15:36:33 imx6qdl-variscite-som systemd[1]: Starting Hostname Service...
| Sep 11 15:36:33 imx6qdl-variscite-som systemd[296]: systemd-hostnamed.service: PrivateNetwork=yes is configured, but the kernel does not support network namespaces, ignorin>

To enable it needs CONFIG_NAMESPACES=y either.

Same as [1]

[1] https://github.com/Freescale/linux-fslc/pull/35

Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>