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/main.c

@@ -49,7 +49,6 @@
 #include "normalize.h"
 #include "zeroconf.h"
 #include "main_notify.h"
-#include "os_compat.h"
 #include "dirvec.h"
 #include "songvec.h"
 #include "tag_pool.h"
@@ -60,6 +59,14 @@
 
 #include <glib.h>
 
+#include <unistd.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <errno.h>
+#include <string.h>
+#include <pwd.h>
+#include <grp.h>
+
 #ifdef HAVE_LOCALE
 #include <locale.h>
 #endif