libxmlsec: openssl conditional was accidentally inverted

... in commit bf6d1f77420dcc9ece4d9f4eae1e37b427d85c6a.  And it just
happens to not build with clang here.

Change-Id: Ic467abc894b9c98c81fa64cb57eafaa4cfcaa66e
(cherry picked from commit e32f828e2b7a136ac255bfe1cc7e4858fa965e8d)
Reviewed-on: https://gerrit.libreoffice.org/7347
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>