clean up CPP includes

Try to only include headers which are really needed.  We should
particularly check all "headers including other headers".  The
long-term goal is to have a manageable, small API for plugins
(decoders, output) without so many mpd internals cluttering the
namespace.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7319 09075e82-0dd4-0310-85a5-a0d7c8717e4f

src/volume.c

@@ -21,11 +21,13 @@
 #include "conf.h"
 #include "log.h"
 #include "player.h"
-#include "state_file.h"
 #include "gcc.h"
 #include "utils.h"
+#include "ack.h"
 #include "os_compat.h"
 
+#include "../config.h"
+
 #ifdef HAVE_OSS
 #include <sys/soundcard.h>
 #endif