F_DUPFD_CLOEXEC is not universally available

The same issue (and the same patch to the byte) was experienced/proposed
by Vince Weaver.
Signed-off-by: Nmalc <av1474@comtv.ru>

linux-user/syscall.c

@@ -3685,8 +3685,10 @@ static int target_to_host_fcntl_cmd(int cmd)
             return F_SETLEASE;
         case TARGET_F_GETLEASE:
             return F_GETLEASE;
+#ifdef F_DUPFD_CLOEXEC
         case TARGET_F_DUPFD_CLOEXEC:
             return F_DUPFD_CLOEXEC;
+#endif
         case TARGET_F_NOTIFY:
             return F_NOTIFY;
 	default: