Bug#1041339: fenix-plugins: Please provide mixer plugin

[Thomas Uhle]

Control: block -1 by 1041336

Dear maintainers,

please find attached a simple patch to enable building the mixer plugin 
as well.  Please note that it only applies cleanly if the fixes from 
ticket #1041336 has already been applied before.

Thank you in advance for considering these changes!

Best regards,

Thomas Uhle