tests: benchmarks: sys_kernel: 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/sys_kernel/src/syskernel.h

@@ -22,8 +22,8 @@
 #endif
 
 
-K_THREAD_STACK_EXTERN(thread_stack1);
-K_THREAD_STACK_EXTERN(thread_stack2);
+K_THREAD_STACK_DECLARE(thread_stack1, STACK_SIZE);
+K_THREAD_STACK_DECLARE(thread_stack2, STACK_SIZE);
 extern struct k_thread thread_data1;
 extern struct k_thread thread_data2;