X3DImporter: Add virtual destructors to some classes which already have virtual functions

code/FIReader.hpp

@@ -62,6 +62,7 @@ namespace Assimp {
 
 struct FIValue {
     virtual const std::string &toString() const = 0;
+    virtual ~FIValue() {}
 };
 
 struct FIStringValue: public FIValue {
@@ -121,6 +122,7 @@ struct FICDATAValue: public FIStringValue {
 
 struct FIDecoder {
     virtual std::shared_ptr<const FIValue> decode(const uint8_t *data, size_t len) = 0;
+    virtual ~FIDecoder() {}
 };
 
 struct FIQName {