fix compilation error on clang

alib2data/src/label/NextLabel.h

@@ -36,6 +36,8 @@ class NextLabel : public VisitableLabelBase::const_visitor_type, public Visitabl
 	void Visit(void*, UniqueLabel& label) const;
 public:
+	NextLabel() {}
+
 	/**
 	 * Prints XML representation of String to the output stream.
 	 * @param string String to print

alib2data/src/primitive/NextPrimitive.h

@@ -35,6 +35,8 @@ class NextPrimitive : public VisitablePrimitiveBase::const_visitor_type, public
 	void Visit(void*, Bool& primitive) const;
 public:
+	NextPrimitive() {}
+
 	/**
 	 * Prints XML representation of String to the output stream.
 	 * @param string String to print