Add missing import for newer libstdc++.

Taken from upstream:

  https://github.com/qt/qtwayland/commit/1aa6ec2c778504d96543f6cdc2b9199a7b066fc1

diff --git a/tests/auto/client/shared/corecompositor.cpp b/tests/auto/client/shared/corecompositor.cpp
index 5c6c83baa..fa9b7662a 100644
--- a/tests/auto/client/shared/corecompositor.cpp
+++ b/tests/auto/client/shared/corecompositor.cpp
@@ -27,6 +27,7 @@
 ****************************************************************************/
 
 #include "corecompositor.h"
+#include <thread>
 
 namespace MockCompositor {
 
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<tr><td><span title='2019-11-26 00:16:36 +0100'>2019-11-26</span></td><td><a href='/guix/commit/nix/boost/format?id=3a4d5ddd322c7daf0a8c83ca2c87c3fc38b19f0b'>daemon: boost::format: Fix typo "referred".</a><span class='msg-avail'>...<span class='msg-tooltip'>* nix/boost/format/exceptions.hpp (too_few_args): Fix typo.
  (too_many_args): Fix typo.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Vagrant Cascadian</td></tr>
4 10:52:52 +0200'>2020-03-04</span></td><td><a href='/guix/commit/gnu/packages/patches/zziplib-CVE-2018-16548.patch?id=790b66e34fd81cdda246361303072065e6339c97'>gnu: zziplib: Fix CVE-2018-16548.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/compression.scm (zziplib)[replacement]: New field.
(zziplib/fixed): New private variable.
* gnu/packages/patches/zziplib-CVE-2018-16548.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it.
</span></span></td><td>Efraim Flashner</td></tr>