journal-gateway: use MHD_USE_POLL_INTERNAL_THREAD instead of MHD_USE_POLL

journal-gateway: use MHD_USE_POLL_INTERNAL_THREAD instead of MHD_USE_POLL

The option MHD_USE_THREAD_PER_CONNECTION requires MHD_USE_POLL_INTERNAL_THREAD
since libmicrohttpd-0.9.53.
If MHD_USE_POLL is used instead of MHD_USE_POLL_INTERNAL_THREAD, then
the library outputs the following warning:
```
Warning: MHD_USE_THREAD_PER_CONNECTION must be used only with
MHD_USE_INTERNAL_POLLING_THREAD. Flag MHD_USE_INTERNAL_POLLING_THREAD was added.
Consider setting MHD_USE_INTERNAL_POLLING_THREAD explicitly.
```
The option MHD_USE_POLL_INTERNAL_THREAD is defined as
`MHD_USE_POLL_INTERNAL_THREAD = MHD_USE_POLL | MHD_USE_INTERNAL_POLLING_THREAD,`
So, let's use MHD_USE_POLL_INTERNAL_THREAD instead of MHD_USE_POLL.