[zxio] Call newly added zxio_destroy.

In fxr/359358, a new zxio function was introduced to decouple the
closing of connection from the closing of the client-end handle,
in order to resolve a race condition when closing zxio in the precense
of concurrent operations.

Change-Id: I206737e1b46d67fb253290da48de7e0d345a1d89
diff --git a/src/util/os_dirent_fuchsia.cpp b/src/util/os_dirent_fuchsia.cpp
index af467f8..4b91685 100644
--- a/src/util/os_dirent_fuchsia.cpp
+++ b/src/util/os_dirent_fuchsia.cpp
@@ -51,6 +51,7 @@
       }
       if (dir_init) {
          zxio_close(&io_storage.io);
+         zxio_destroy(&io_storage.io);
       }
    }