tests: benchmarks: footprint: Migrate to K_THREAD_STACK_DECLARE

This commit updates all deprecated `K_THREAD_STACK_EXTERN` macro usages
to use the `K_THREAD_STACK_DECLARE` macro instead.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

tests/benchmarks/footprints/src/footprint.h

@@ -10,13 +10,13 @@
 #include <zephyr/kernel.h>
 #include <zephyr/app_memory/app_memdomain.h>
 
-K_THREAD_STACK_EXTERN(my_stack_area);
-K_THREAD_STACK_EXTERN(my_stack_area_0);
+#define STACK_SIZE	512
+
+K_THREAD_STACK_DECLARE(my_stack_area, STACK_SIZE);
+K_THREAD_STACK_DECLARE(my_stack_area_0, STACK_SIZE);
 extern struct k_thread my_thread;
 extern struct k_thread my_thread_0;
 
-#define STACK_SIZE	512
-
 #ifdef CONFIG_USERSPACE
 #define FP_DMEM		K_APP_DMEM(footprint_mem_partition)
 #define FP_BMEM		K_APP_BMEM(footprint_mem_partition)