[Pkg-mpd-maintainers] Bug#993359: Update systemd unit to raise memlock limit for io_uring support

[MichaIng]

Package: mpd
Version: 0.22.6-1

Hey guys,

with v0.22, MPD introduced support for io_uring kernel interface, which 
has been enabled for the Debian package with v0.22.3-1 as well.

The problem is that for making use of it, the default memlock limit on 
Debian of 64 KiB is too small, and the daemon hence throws an exception, 
using a fallback:
-------
Failed to initialize io_uring: io_uring_queue_init() failed: Cannot 
allocate memory
-------

The memlock limit can be raised via systemd unit, which has been done 
upstream: 
https://github.com/MusicPlayerDaemon/MPD/blob/v0.22.x/systemd/system/mpd.service.in#L19-L20
But this change obviously wasn't applied to the Debian package.

Applying this directive solves the exception, so I suggest to apply it 
to the Debian package as well.

Further references:
- https://github.com/MusicPlayerDaemon/MPD/issues/972
- https://github.com/MichaIng/DietPi/issues/4690

Best regards,

Micha