Related: fdo#35105 initially map HEADING to ATK_ROLE_HEADING

gets overwritten anyway, but its less confusing this way

Change-Id: Ib7da2b7787afa7804496337358f0ef6fc7d42b3e
Reviewed-on: https://gerrit.libreoffice.org/7834
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>