[LOG4J2-3601] log4j-slf4j2-impl: depend on log4j-core scope=runtime

[afs]

https://issues.apache.org/jira/browse/LOG4J2-3601

This makes the dependency structure the same between log4j-slf4j-impl and log4j-slf4j2-impl.

Then user applications only need a dependency on log4j-slf4j2-impl and it is then the same pattern as for log4j-slf4j-impl.