[wwwdocs] Add <thread> to libstdc++ header dependency changes

[Jonathan Wakely]

Committed to wwwdocs.

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commit ed1a8d0621f5f5dee4d869393b9155f475d360f7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Mar 4 20:51:52 2021 +0000

    Add <thread> to libstdc++ header dependency changes

diff --git a/htdocs/gcc-11/porting_to.html b/htdocs/gcc-11/porting_to.html
index 83227c74..4d27c163 100644
--- a/htdocs/gcc-11/porting_to.html
+++ b/htdocs/gcc-11/porting_to.html
@@ -112,6 +112,9 @@ be included explicitly when compiled with GCC 11:
   (for <code>std::pair</code>, <code>std::tuple_size</code>,
   <code>std::index_sequence</code> etc.)
 </li>
+<li> <code><thread></code>
+  (for members of namespace <code>std::this_thread</code>.)
+</li>
 </ul>
 
 <h3 id="Old-iostream-Members">Old iostream Members</h3>