samples: net: echo*: Increase stack size

Increase stack size to 1200 for echo and echo_async sample apps
(following a similar increase done to echo_async_select previously).
With default stack size, the apps no longer run (crash QEMU) with
Zephyr 2.4-pre.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
2 files changed