Fix mismatched tag class vs struct

See friend declaration below. That's the reason why we decided to use
'class' almost everywhere.

Change-Id: Icd8ba99528eac28721b403cac0a992ee4d367265
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/cpptools/cppoverviewmodel.h

@@ -32,8 +32,9 @@
 
 namespace CppTools {
 
-struct SymbolItem : public Utils::TreeItem
+class SymbolItem : public Utils::TreeItem
 {
+public:
     SymbolItem() = default;
     SymbolItem(CPlusPlus::Symbol *symbol) : symbol(symbol) {}