Prevent the default variants to be used as fallback

[Vampire]

Currently if you for example request Windows/x86, as no variant is configured for it, you get the default variants with the empty jars. With this additional attribute change, you instead get an error that no matching variant is found.

@jjohannes what do you think?

[Vampire]

I've just seen this is also done in the LWJGL example in the Gradle docs: https://docs.gradle.org/current/userguide/component_metadata_rules.html#adding_variants_for_native_jars