libc/aio/aio.h is empty, but add some protection anyway in case it is ever used in the future

libc/aio/aio.h

@@ -42,6 +42,8 @@
 
 #include <nuttx/config.h>
 
+#ifdef CONFIG_FS_AIO
+
 /****************************************************************************
  * Pre-processor Definitions
  ****************************************************************************/
@@ -67,4 +69,5 @@ extern "C"
  * Public Function Prototypes
  ****************************************************************************/
 
+#endif /* CONFIG_FS_AIO */
 #endif /* __LIBC_AIO_AIO_H */