Fix kfilemetadata_exiv2extractor build on macOS
When building on macOS (with exiv2 provided by homebrew), the exiv2 headers depend on libssh/libssh.h, but since this is in /usr/local/include it is not found by default.
When building on macOS (with exiv2 provided by homebrew), the exiv2 headers depend on libssh/libssh.h, but since this is in /usr/local/include it is not found by default.