Add a message to suggest checking crate-type

[LegNeato]

This was my issue and it was unclear the cause. I am not sure if this is the only case where this error shows up, hence the wording. Open to better wording if you have it.

[Firestar99]

Verify that crate-type is set correctly, eg. crate-type = ["lib", "dylib"] within the [lib] section of your shader's Cargo.toml.

I know technically "dylib" is sufficient, but without lib I'm concerned that if the crate is also used on the CPU, it switches to dynamic linking instead of static linking and inlining + LTO can't do much then. But take this with a grain of salt, not too familiar with this topic.