xmlChildElementCount is only available in libxml2 >= 2.7.3

fixes build error Undefined symbols: _xmlChildElementCount

Change-Id: I9687971912b4d0ae55bf1d9ad987fd339fcbec7e
Reviewed-on: https://gerrit.libreoffice.org/9788
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>