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/dirvec.c

@@ -3,9 +3,11 @@
 #include "utils.h"
 #include "path.h"
 
-#include <string.h>
 #include <glib.h>
 
+#include <assert.h>
+#include <string.h>
+
 static GMutex *nr_lock = NULL;
 
 static size_t dv_size(const struct dirvec *dv)