Exclude a few more things from published crates

[kalev]

The bundled C library in the ext directory is only used by internal libqoi that isn't published on crates.io, and test_gen.rs tests libqoi (which again isn't published).

The pdf doc isn't used for the build of the published crate either, so exclude this as well.

Pointed out by Fabio Valentini in https://bugzilla.redhat.com/show_bug.cgi?id=2233787

[aldanor]

Agreed re: the doc folder.

Re: libqoi and test_gen.rs, I'm unsure about that, if you remove those, you won't be able to simply run cargo test because of

target/debug/build/libqoi-ec49f992f9130877/out/qoi.c:1:10: fatal error: 'qoi.h' file not found

test_gen.rs tests libqoi (which again isn't published).

Not quite - test_gen.rs tests that the following roundtrips successfully for a bunch of generated samples:

• qoi-rust <--> qoi-rust
• qoi-rust <--> qoi.h