Fix const modifier in getHistoricString()

Source/Processors/Channel/InfoObjects.cpp

@@ -56,7 +56,7 @@ String NodeInfoBase::getCurrentNodeName() const
 HistoryObject::~HistoryObject()
 {}
 
-String HistoryObject::getHistoricString()
+String HistoryObject::getHistoricString() const
 {
 	return m_historicString;
 }

Source/Processors/Channel/InfoObjects.h

@@ -71,7 +71,7 @@ protected:
 public:
     virtual ~HistoryObject();
 	/** Returns the historic string */
-	String getHistoricString();
+	String getHistoricString() const;
 	/** Adds a new entry in the historic string*/
 	void addToHistoricString(String entry);