removed os_compat.h

Only include headers which are really needed.  os_compat.h aimed to
make MPD easily portable, but was never actually made portable.

src/playlist.c

@@ -36,10 +36,15 @@
 #include "ack.h"
 #include "idle.h"
 #include "list.h"
-#include "os_compat.h"
 
 #include <glib.h>
 
+#include <assert.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+#include <stdlib.h>
+
 #define PLAYLIST_STATE_STOP		0
 #define PLAYLIST_STATE_PLAY		1