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='2021-12-13 17:48:25 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/packages/perl-maths.scm?id=8394619baceb118df92e355377fd543bb1aa501a'>gnu: Simplify package inputs.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
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