gexiv2: Update to 0.12.1

gexiv2 0.12.1 - 23 May 2020

* Support macOS dylib versioning
* Improve compatibility with current exiv2 development versions
* Fix compiling on windows
* Fix issue with problematic symbol visibility of exiv2
* Fix issues with tests overwriting environment variables
* Fix exception escaping on division by zero
* Fix overflow in altitude fraction conversion
* Fix crash with experimental GIO support
* Documentation fixes
* Add gexiv2_metadata_update_gps_info which will not remove all other GPS
  information

Bugs fixed in this release:
- https://gitlab.gnome.org/GNOME/gexiv2/issues/41
- https://gitlab.gnome.org/GNOME/gexiv2/issues/42
- https://gitlab.gnome.org/GNOME/gexiv2/issues/45
- https://gitlab.gnome.org/GNOME/gexiv2/issues/52
- https://gitlab.gnome.org/GNOME/gexiv2/issues/53
- https://gitlab.gnome.org/GNOME/gexiv2/issues/54
- https://gitlab.gnome.org/GNOME/gexiv2/issues/55

(nia)