Install the missing deprecated header files

Deprecated Qt header files generated by syncqt were missing in
installation package of Qt, after migration to cpp based syncqt.
Restore them.

Note: we didn't receive bugs since 6.5 release so far. I would check
the existing files and remove them completely in 6.7. See QTBUG-115029.

Pick-to: 6.5 6.6
Fixes: QTBUG-112956
Change-Id: I2e5375ee0dbd87a76135594cd489bb67f6d3456d
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

src/tools/syncqt/main.cpp

@@ -1467,6 +1467,7 @@ public:
                    << "#include <" << replacement << ">\n"
                    << "#endif\n";
             writeIfDifferent(m_commandLineArgs->includeDir() + '/' + it->first, buffer.str());
+            m_producedHeaders.insert(it->first);
         }
         return true;
     }