[FLAC] Pack hasiXML/hasBEXT with scanned to save padding

Per review feedback on #1364: moving the two new bool flags to the end
of FilePrivate, next to the existing `scanned` bool, lets the compiler
coalesce the three bytes into the same trailing padding slot.  Saves one
machine word per FLAC::File instance versus placing the flags mid-struct
between bextData and the List<MetadataBlock*>.

Pure layout change, no behaviour difference.  Test suite still green.

taglib/flac/flacfile.cpp

@@ -72,13 +72,13 @@ public:
   ByteVector xiphCommentData;
   String iXMLData;
   ByteVector bextData;
-  bool hasiXML { false };
-  bool hasBEXT { false };
   List<FLAC::MetadataBlock *> blocks;
 
   offset_t flacStart { 0 };
   offset_t streamStart { 0 };
   bool scanned { false };
+  bool hasiXML { false };
+  bool hasBEXT { false };
 };
 
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