log4j-docgen: `log4j-1.2-api` layouts are skipped

[vy]

log4j-1.2-api layouts are skipped while generating plugin descriptors.

Reproduction

1. Switch to the directory containing apache/logging-log4j2
2. ./mvnw install -DskipTests
3. ./mvnw site
4. Observe that target/plugin-descriptors/phase2/log4j-1.2-api-plugins.xml is missing layouts