pulse: clean up includes

Don't include output_api.h - this is not an output plugin.  Added
missing explicit conf.h and string.h includes.

src/mixer/pulse_mixer.c

@@ -1,10 +1,12 @@
-#include "../output_api.h"
-#include "../mixer_api.h"
+#include "mixer_api.h"
+#include "conf.h"
 
 #include <glib.h>
 #include <pulse/volume.h>
 #include <pulse/pulseaudio.h>
 
+#include <string.h>
+
 struct pulse_mixer {
 	struct mixer base;
 	char *server;