samples: tflite-micro: hello_world: Increase main stack size

This commit increases the main stack size to 2048 since the current
stack size is insufficient for the `qemu_x86` board (the default stack
size is 1024 for that board) and results in a stack overflow in the
main thread.

Note that this is required because the newlib full variant, which is
used by this sample, included in the Zephyr SDK 0.13.2 now has an
increased stack usage requirement due to enabling the C99 format
specifier support.

For more details, refer to the issue #40469 [1].

[1] https://github.com/zephyrproject-rtos/zephyr/pull/40469#issuecomment-977854095



Signed-off-by: Stephanos Ioannidis <root@stephanos.io>