fix compile for change to boost 1.53.0 declaring smart pointer operator bool as explicity for C++11 compilers

Change-Id: If2c3ad68b2ffea645a9f2035cd802553edc0ee79
Reviewed-on: https://gerrit.libreoffice.org/2064Tested-by: LibreOffice gerrit bot <gerrit@libreoffice.org>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>