samples: Bluetooth: Mesh: add main stack for mesh provisioner

The current main stack size is not enough for any platform.
This commit stack size was checked for nrf52840 and nrf5340.
Everything works.

Signed-off-by: Aleksandr Khromykh <aleksandr.khromykh@nordicsemi.no>

samples/bluetooth/mesh_provisioner/prj.conf

@@ -1,5 +1,5 @@
 #CONFIG_INIT_STACKS=y
-CONFIG_MAIN_STACK_SIZE=1408
+CONFIG_MAIN_STACK_SIZE=2048
 CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=2048
 # The Bluetooth API should not be used from a preemptive thread:
 CONFIG_MAIN_THREAD_PRIORITY=-2