preprocessor: fix bug in merge of rmExtraExo branch

preprocessor/ExprNode.hh

@@ -34,6 +34,7 @@ using namespace std;
 class DataTree;
 class VariableNode;
 class BinaryOpNode;
+class SymbolTable;
 
 typedef class ExprNode *expr_t;
 

preprocessor/ExternalFunctionsTable.hh

@@ -28,6 +28,7 @@ using namespace std;
 #include <map>
 
 #include "SymbolTable.hh"
+class SymbolTable;
 
 enum ExternalFunctionSetOrNot
   {