f81c28be replaced org.jivesoftware.util.Log with org.slf4j.Logger.
This commit partially reverts this.

Apparently the build process produces an additional startup.jar, which contains the ServerStarter class. However, it doesn't add slf4j-api.jar to the classpath or manifest.
Because Openfire's classpath, lib folder and build structure is pretty confusing (e.g. slf4j-api.jar gets merged into a new openfire.jar, there's an extra JiveClassLoader, etc...), it's hard to solve this in a reasonable way at the moment.