Silence -Werror=implicit-fallthrough= (upcoming GCC 8)

Earlier GCC already require a similar workaround to silence -Werror=return-type
("control reaches end of non-void function") a few lines further down, so it's
rather consequential that GCC now also warns about the potential fall-through
here (i.e., for these warnings, GCC apparently assumes that an enum can take on
all of the values from its range of values, not just those denoted by an
enumerator).

Change-Id: I1537a7f6975f900861225ee7a521366a5e57046a

xmlreader/source/xmlreader.cxx

@@ -134,7 +134,7 @@ XmlReader::Result XmlReader::nextItem(Text reportText, Span * data, int * nsId)
             return handleSkippedText(data, nsId);
         case Text::Raw:
             return handleRawText(data);
-        case Text::Normalized:
+        default: // Text::Normalized
             return handleNormalizedText(data);
         }
     case State::StartTag: