Bug#924862: shaderc isn't needed

[Shmerl <shtetldik+shmerl@gmail.com>]

I asked mpv developers, and apparently libplacebo indeed now can handle shader

compilation without need for shaderc. So this seems to be not even needed at all

on Linux.

See: https://github.com/mpv-player/mpv/issues/12982

Regards,

Shmerl.