Core: Correct declaration of Context debug output function

Amends 104ea4acc.

Hidden friends don't work when declared in the wrong class.
This here happened to go unnoticed as it was nowhere used.

Change-Id: Ie26d5c777c85e143a6b7189ccbd87aedd2502df0
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/icontext.h

@@ -41,6 +41,8 @@ public:
     void add(Utils::Id c) { d.append(c); }
     bool operator==(const Context &c) const { return d == c.d; }
 
+    friend CORE_EXPORT QDebug operator<<(QDebug debug, const Core::Context &context);
+
 private:
     QList<Utils::Id> d;
 };
@@ -64,8 +66,6 @@ public:
                        const Context &context,
                        const HelpItem &contextHelp = {});
 
-    friend CORE_EXPORT QDebug operator<<(QDebug debug, const Core::Context &context);
-
 protected:
     Context m_context;
     QPointer<QWidget> m_widget;