Do not depend on libxml include in widely included file

...and hope xmTextWriterPtr expanding to struct _xmlTextWriter * is a stable feature
of the libxml2 API

Change-Id: Ic5076df5a8b33f71c605b3aef737a99ea96b1753

include/editeng/editobj.hxx

@@ -33,8 +33,6 @@
 
 #include <vector>
 
-#include <libxml/xmlwriter.h>
-
 class SfxItemPool;
 class SfxStyleSheetPool;
 class SvxFieldItem;
@@ -148,7 +146,7 @@ public:
 #if DEBUG_EDIT_ENGINE
     void Dump() const;
 #endif
-    void dumpAsXml(xmlTextWriterPtr pWriter) const;
+    void dumpAsXml(struct _xmlTextWriter * pWriter) const;
 };
 
 #endif // INCLUDED_EDITENG_EDITOBJ_HXX

svx/Library_svxcore.mk

@@ -87,7 +87,6 @@ $(eval $(call gb_Library_use_externals,svxcore,\
 	icu_headers \
 	mesa_headers \
 	glew \
-	libxml2 \
 ))
 
 ifeq ($(OS),MACOSX)