Bug#1041336: fenix-plugins: build improvements

[Thomas Uhle]

Dear maintainers,

please find attached two fixes for the packages fenix-plugin-mpeg and 
fenix-plugins.

With 05-include-SDL-mixer.patch, the missing '#include <SDL_mixer.h>' 
is inserted into mpeg-1.0/mpeg.c.

With 06-missing-braces.patch, some missing braces are added to avoid 
confusion because of misleading indentation.

I think it would also be benefitial if fenix-plugins would build-depend 
on libfreetype-dev instead of the deprecated libfreetype6-dev.  Can you 
please change this as well.

Thank you in advance for considering the changes!

Best regards,

Thomas Uhle