test/kernel: Initialize stack_sem in obj_validation test

GCC 14.3 is not happy when this variable is left uninitialized, although
we don't actually care about the contents, just the address.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

tests/kernel/mem_protect/obj_validation/src/main.c

@@ -83,7 +83,7 @@ void object_permission_checks(struct k_sem *sem, bool skip_init)
  */
 ZTEST(object_validation, test_generic_object)
 {
-	struct k_sem stack_sem;
+	struct k_sem stack_sem = {};
 
 	/* None of these should be even in the table */
 	zassert_false(test_object(&stack_sem, -EBADF));