Upgrade to 0.27.2

See previous communication on the ML: https://alioth-lists.debian.net/pipermail/pkg-kde-talk/2019-February/002907.html https://alioth-lists.debian.net/pipermail/pkg-kde-talk/2019-July/002918.html

The main changes are upgrading to upstream's second bugfix release of version 0.27 of the library and enabling xmp again. I'd claim that xmp support is important for most projects using exiv2 for metadata handling, as this is nowadays the primary method for keeping metadata after image-capturing. And I know that darktable would be broken without it (they save image processing info in xmp) and digikam would be severally crippled.