Disable building exiv2 extractor until some upstream issues are fixed

Mainly, after bumping the C++ standard we compile with to 17, the build on FreeBSD fails on the CI:

18:10:12 In file included from /usr/local/include/exiv2/metadatum.hpp:38: 18:10:12 /usr/local/include/exiv2/value.hpp:63:17: error: no template named 'auto_ptr' in namespace 'std'; did you mean simply 'auto_ptr'? 18:10:12 typedef std::auto_ptr AutoPtr; 18:10:12 ^~~~~ 18:10:12 /usr/local/include/exiv2/config.h:109:3: note: 'auto_ptr' declared here 18:10:12 using auto_ptr = std::unique_ptr; 18:10:12 ^