OOXMLFastContextHandler has a deleted copy assignment op

...for whatever reason, ever since the file's introduction with
359fd901 "INTEGRATION: CWS xmlfilter02".  Make
the copy ctor expliticly defaulted to avoid -Werror=deprecated-copy with GCC
trunk towards GCC 9.

Change-Id: I7785dfb073767ba963ceec682cfbdfe192eed69b
Reviewed-on: https://gerrit.libreoffice.org/58040
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

writerfilter/source/ooxml/OOXMLFastContextHandler.hxx

@@ -46,6 +46,8 @@ public:
 
     explicit OOXMLFastContextHandler(OOXMLFastContextHandler * pContext);
 
+    OOXMLFastContextHandler(OOXMLFastContextHandler const &) = default;
+
     virtual ~OOXMLFastContextHandler() override;
 
     // css::xml::sax::XFastContextHandler: