fix display of image orientation

Summary:
The name of the orientation property
for baloo/kfilemetadata was false, leading to
an always empty image orientation field.

Test Plan:
Look at an image in Dolphin details view
which has the exiv orientation tag set. Orientation
column is not empty.

Reviewers: #dolphin, ngraham

Reviewed By: #dolphin, ngraham

Subscribers: ngraham, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D18008