msh: use list_memheap when RT_USING_MEMHEAP_AS_HEAP is enabled

There is no list_mem and only list_memheap when RT_USING_MEMHEAP_AS_HEAP
is enabled.

components/finsh/msh_cmd.c

@@ -281,7 +281,11 @@ int cmd_free(int argc, char** argv)
 {
     extern void list_mem(void);
 
+#ifdef RT_USING_MEMHEAP_AS_HEAP
+    list_memheap();
+#else
     list_mem();
+#endif
     return 0;
 }
 FINSH_FUNCTION_EXPORT_ALIAS(cmd_free, __cmd_free, Show the memory usage in the system.);