QmlDesigner: Move includes to string layout

To fix build on MacOS.

Change-Id: Ie697ee7c5d0f57f0ba9735b36c704196ef7e5a4f
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
Reviewed-by: Burak Hancerli <burak.hancerli@qt.io>

src/libs/utils/smallstring.h

@@ -16,14 +16,8 @@
 #include <QStringEncoder>
 
 #include <algorithm>
-#include <charconv>
-#include <cmath>
-#include <cstdlib>
-#include <climits>
-#include <cstring>
 #include <initializer_list>
 #include <numeric>
-#include <string>
 #include <unordered_map>
 #include <utility>
 #include <vector>

src/libs/utils/smallstringlayout.h

@@ -8,9 +8,14 @@
 #include <QtGlobal>
 
 #include <algorithm>
+#include <charconv>
+#include <climits>
+#include <cmath>
 #include <cstdint>
+#include <cstdlib>
 #include <cstring>
 #include <memory>
+#include <string>
 #include <type_traits>
 
 QT_WARNING_PUSH