[Fix] Add bpe_simple_vocab_16e6.txt.gz to release (open-mmlab#3386)

## Motivation

open-mmlab#3383

## Modification

- Add bpe_simple_vocab_16e6.txt.gz to `MANIFEST.in`

Author: xiexinch

MANIFEST.in

@@ -1,5 +1,5 @@
 include requirements/*.txt
 include mmseg/.mim/model-index.yml
-include mmaction/.mim/dataset-index.yml
+include mmseg/utils/bpe_simple_vocab_16e6.txt.gz
 recursive-include mmseg/.mim/configs *.py *.yaml
 recursive-include mmseg/.mim/tools *.py *.sh