Utils: Fix build on Windows

Amends d97c431a00.

Change-Id: I6a806048772a1e94a20d391723ad5c41886b601e
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/layoutbuilder.h

@@ -50,7 +50,7 @@ public:
 
     ~LayoutBuilder();
 
-    class LayoutItem
+    class QTCREATOR_UTILS_EXPORT LayoutItem
     {
     public:
         LayoutItem();