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='2021-10-14 00:24:34 +0200'>2021-10-14</span></td><td><a href='/guix/commit/.gitattributes?id=f8b10932c86cf4c1657a67e8c484d76f757b64bd'>Add git configuration templates to improve diff hunk header detection.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a follow-up to commit 9fc8ae4171e5da4939a64fc6d684c8b9d85bbe84, which
missed two hunks from &lt;https://issues.guix.gnu.org/50363&gt;.

Reported by hackeryarn on #guix.

* .gitattributes, etc/git/gitconfig: New files.

Signed-off-by: Marius Bakke &lt;marius@gnu.org&gt;
</span></span></td><td>Sarah Morgensen</td></tr>