only include libc-tsd.h if we have threads support

libc/inet/rpc/rpc_thread.c

@@ -2,11 +2,12 @@
 #include <features.h>
 #include <stdio.h>
 #include <assert.h>
-#include <bits/libc-tsd.h>
 #include "rpc_private.h"
 
 #ifdef __UCLIBC_HAS_THREADS__
 
+#include <bits/libc-tsd.h>
+
 /* Variable used in non-threaded applications or for the first thread.  */
 static struct rpc_thread_variables __libc_tsd_RPC_VARS_mem;
 static struct rpc_thread_variables *__libc_tsd_RPC_VARS_data =