Added missing forward declaration.

lib/pddlparse/include/pddlparse/NormalizedASTForward.h

@@ -82,6 +82,7 @@ struct Description;
 using DescriptionPointer = std::unique_ptr<Description>;
 struct Domain;
 using DomainPointer = std::unique_ptr<Domain>;
+struct InitialState;
 struct Problem;
 using ProblemPointer = std::unique_ptr<Problem>;
 enum class Requirement;