package/tovid: switch from mplayer to mpv

The mplayer package will be removed from buildroot, tovid can use mpv
instead. Support for mpv was added upstream:
https://github.com/tovid-suite/tovid/commit/05cbce2ea130e50f6bf1599816dd0d7bf3ace91a



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>