configure_hotkey: Make IsUsedKey() a const member function

This doesn't actually modify instance state of the dialog, so this can
be made const.

src/citra_qt/configuration/configure_hotkeys.h

@@ -42,8 +42,8 @@ signals:
 
 private:
     void Configure(QModelIndex index);
-    bool IsUsedKey(QKeySequence key_sequence);
-    QList<QKeySequence> GetUsedKeyList();
+    bool IsUsedKey(QKeySequence key_sequence) const;
+    QList<QKeySequence> GetUsedKeyList() const;
 
     std::unique_ptr<Ui::ConfigureHotkeys> ui;