graphics/librsvg2: fix build with newer libxml2
- explicitly #include <libxml/tree.h>, probably from <libxml/SAX.h> before
- account for changed xmlStructuredErrorFunc typedef in <libxml/xmlerror.h>
While here, cosmetic ordering and fix gtk-doc typo; bump PORTREVISION
for that.
PR: 279705
Co-authored-by: vvd