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/command.c

@@ -38,9 +38,11 @@
 #include "client.h"
 #include "tag_print.h"
 #include "path.h"
-#include "os_compat.h"
 #include "idle.h"
 
+#include <assert.h>
+#include <time.h>
+
 #define COMMAND_STATUS_VOLUME           "volume"
 #define COMMAND_STATUS_STATE            "state"
 #define COMMAND_STATUS_REPEAT           "repeat"