Fix feature flags for externally defined classes

[madsmtm]

In https://github.com/madsmtm/icrate-generated/commit/fe934af15ee6bed27b6cf72d3c40cf22ffbc18db, I noticed that some feature-flags were not correctly identified, it seems that forward declarations like @class MyClass is picked up as the definition of the class instead, and we take the framework name from that.