Include libc_private.h for the declaration of __isthreaded instead of

relying on namespace pollution in stdio.h.

MFC after:	3 days

lib/libc/net/nsdispatch.c

@@ -86,6 +86,7 @@ __FBSDID("$FreeBSD$");
 #include <syslog.h>
 #include <unistd.h>
 #include "un-namespace.h"
+#include "libc_private.h"
 #ifdef NS_CACHING
 #include "nscache.h"
 #endif