Add defmt feature to embedded-can Cargo.toml

[dymk]

In order for the defmt dep to be used, it needs to be enabled by a feature (in this case, of the same name). As far as I can tell, simply having the dependency be optional does not create an implicit feature to enable it.

[Dirbaio]

simply having the dependency be optional does create an implicit feature to enable it :)

[dymk]

Oops, that's embarrassing - I was on v0.4.1 of embedded-can which doesn't have the defmt feature, I mistook that as "optional dep does not define the feature". My bad!

I see this is something others have ran into in the issue tracker - I'll toss another vote out there for a release of embedded-can, looks like it's been a while since there's been one. For now, I'll use git as the package source in my cargo.toml.