samples/mgmt/smp_svr: Increase main stack size
samples/mgmt/smp_svr: Increase main stack size

Some build variants will fail runtime for certain boards (e.g. nRF52832)
due to stack overflow. Avoid this by increasing the stack size.

Signed-off-by: Håkon Øye Amundsen <4238ca2ac515887d8a25d5e30932b12519303bf7@nordicsemi.no>

zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr