Add INIT_FS_EXPORT for RAM file system

components/dfs/filesystems/ramfs/dfs_ramfs.c

@@ -414,6 +414,7 @@ int dfs_ramfs_init(void)
 
     return 0;
 }
+INIT_FS_EXPORT(dfs_ramfs_init);
 
 struct dfs_ramfs* dfs_ramfs_create(rt_uint8_t *pool, rt_size_t size)
 {