FIX: Exprtk wrapper now compatible with VS 2019

src/General/ExprtkWrapper.cpp

@@ -11,6 +11,7 @@
 #include "ExprtkWrapper.h"
 #include "ExprtkWrapperSerialization.h"
 #include "exceptions.h"
+#include <string>
 
 BOOST_CLASS_EXPORT_IMPLEMENT(ExprtkWrapper);
 

src/General/ExprtkWrapper.h

@@ -10,6 +10,7 @@
 
 #include <memory>
 #include <vector>
+#include <string>
 
 #include "../settings.h"