samples: hello_world: set minimum flash requirement to 16K

Default in Twister is 32K, but 16K is a more "reasonable" goal and allows
to build hello_world against some of the most constrained boards in-tree.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>

samples/hello_world/sample.yaml

@@ -4,6 +4,7 @@ sample:
   name: hello world
 common:
   min_ram: 2
+  min_flash: 16
   tags: introduction
   integration_platforms:
     - native_sim