include: sys: sys_heap: fix ifdef guards

Ifdef guard of CONFIG_SYS_HEAP_RUNTIME_STATS prevents using IS_ENABLED
macro, preferred way of guarding code. This commit fixes that by
removing ifdef macro.

Signed-off-by: Evgenii Kosenko <Evgenii.Kosenko@silabs.com>

Author: kedMertens

include/zephyr/sys/sys_heap.h

@@ -73,8 +73,6 @@ struct z_heap_stress_result {
  * @{
  */
 
-#ifdef CONFIG_SYS_HEAP_RUNTIME_STATS
-
 /**
  * @brief Get the runtime statistics of a sys_heap
  *
@@ -96,8 +94,6 @@ int sys_heap_runtime_stats_get(struct sys_heap *heap,
  */
 int sys_heap_runtime_stats_reset_max(struct sys_heap *heap);
 
-#endif
-
 /** @brief Initialize sys_heap
  *
  * Initializes a sys_heap struct to manage the specified memory.