Fix build with exiv2-0.27.1

Summary:
Upstream is moving things around between headers. While they are busy doing that, they recommend including exiv2.hpp.

```
lib/imagemetainfomodel.cpp:293:40: error: expected unqualified-id before ‘&’ token
             } catch (const Exiv2::Error& error) {
                                        ^
```

Reviewers: #gwenview, ngraham

Reviewed By: #gwenview, ngraham

Subscribers: ngraham, devurandom

Tags: #gwenview

Differential Revision: https://phabricator.kde.org/D21148