Fix -Wimplicit-fallthrough warnings

Add "falls through" comments to quench implicit-fallthrough warnings
which are enabled by -Wextra under GCC 7.
diff --git a/xmlwriter.c b/xmlwriter.c
index 69541b8..eb94e6e 100644
--- a/xmlwriter.c
+++ b/xmlwriter.c
@@ -3754,6 +3754,7 @@
             if (count < 0)
                 return -1;
             sum += count;
+            /* Falls through. */
         case XML_TEXTWRITER_DTD_ENTY:
         case XML_TEXTWRITER_DTD_PENT:
             count = xmlOutputBufferWriteString(writer->out, ">");