Export dropwizard in plugin API

gerrit-server:server transitively depends on the dropwizard library
and thus it's included in the plugin API. Exporting it allow plugins
to depend on it in tree build, without explicitly mentioning it in
provided_deps.

Change-Id: Idcda84cd845330e3efb575c9fe9d508d584d7bff

gerrit-plugin-api/BUCK

@@ -28,6 +28,7 @@ java_library(
     '//gerrit-gwtexpui:server',
     '//gerrit-reviewdb:server',
     '//lib:args4j',
+    '//lib/dropwizard:dropwizard-core',
     '//lib:guava',
     '//lib:gwtorm',
     '//lib:jsch',