logback: correct MDC dependency in appender

MDC dependency should be to slf4j, not log4j.
The error has gone unnoticed because we do use
log4j-over-slf4j, but the import should not
be tightly coupled to it.

Target: master
Patch: http://rb.dcache.org/r/6777/
Require-notes: no
Require-book: no
Acked-by: Paul