evolution does not build with latest libxml2

[Sergio de Almeida Lenzi]

FreeBSD 10  and 9.2 does not build
evolution with latest libxml2

====================================
e-cal-backend-caldav.c:1533:28: error: incomplete definition of type
'struct _xmlBuf'
                                  (gchar *) buf->buffer->content,
                                            ~~~~~~~~~~~^
/usr/local/include/libxml2/libxml/tree.h:104:16: note: forward
declaration of 'struct _xmlBuf'
typedef struct _xmlBuf xmlBuf;
               ^
e-cal-backend-caldav.c:1583:28: error: incomplete definition of type
'struct _xmlBuf'
                                  (gchar *) buf->buffer->content,
                                            ~~~~~~~~~~~^
/usr/local/include/libxml2/libxml/tree.h:104:16: note: forward
declaration of 'struct _xmlBuf'
typedef struct _xmlBuf xmlBuf;
======================================