seafile-seahub: update dependency to python-mysqlclient
The `python-mysql` package was updated with PR https://github.com/openwrt/packages/pull/9705
For seahub this was omitted, since the Python dependencies are prefixed
with `python-`, so it was missed during the grep search.
And grepping just for `mysql` yields many results.
Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>