Add a 'jetty' prefix to jetty-specific slf4j build targets

This is in order to not conflict with build targets to be added
to external/slf4j, which will build from source rather than from
the jetty-specific .jar dependencies.

Bug: 29574477
Test: make jetty
Change-Id: I6ce2c4b9b300e8b96446f66e9249953a29302c81