include cleanups (powered by iwyu)

test/read_mixer.cxx

@@ -21,15 +21,11 @@
 #include "mixer/MixerControl.hxx"
 #include "mixer/MixerList.hxx"
 #include "filter/FilterRegistry.hxx"
-#include "pcm/Volume.hxx"
-#include "Main.hxx"
 #include "event/Loop.hxx"
 #include "config/Block.hxx"
 #include "util/PrintException.hxx"
 
 #include <assert.h>
-#include <string.h>
-#include <unistd.h>
 #include <stdlib.h>
 
 const FilterPlugin *