Add Missing Include stddef.h in bt_hal_manager.h (#3517)

bt_hal_manager.h uses size_t type but doesn't include stddef.h

Signed-off-by: Adham Abozaeid <adhsal@amazon.com>

Signed-off-by: Adham Abozaeid <adhsal@amazon.com>
Co-authored-by: Adham Abozaeid <adhsal@amazon.com>

Author: AdhamAbozaeid

libraries/abstractions/ble_hal/include/bt_hal_manager.h

@@ -41,6 +41,7 @@
 #define _BT_HAL_MANAGER_H_
 
 #include <stdint.h>
+#include <stddef.h>
 
 #include "bt_hal_manager_types.h"