fixed compilation error with Apple Clang 7.0.0

(cherry picked from commit dd5b10d9dc8eb7447e37ac30ddd7545897c878b0)

rtengine/metadata.cc

@@ -76,7 +76,8 @@ std::unique_ptr<Exiv2::Image> open_exiv2(const Glib::ustring& fname)
 #endif
         throw Exiv2::Error(error_code, "exiv2: invalid image");
     }
-    return image;
+    std::unique_ptr<Exiv2::Image> ret(image.release());
+    return ret;
 }
 
 } // namespace