Buck: Export joda-time to simplify plugin in tree build

gerrit-server transitively depends on //lib/joda:joda-time, and
thus this dependency was always included in gerrit-plugin-api.
Exporting it allows in tree plugin to also depend on it, without
mentioning it as explicit dependency, which would unnecessarily
complicate standalone build implementation.

This change allows us to simplify Buck in tree and standalone build
implementation for websession-flatfile plugin.

Change-Id: Idfb2401424cd13676a8bb7918278cbcac672dc87

gerrit-plugin-api/BUCK

@@ -40,6 +40,7 @@ java_library(
     '//lib/guice:guice-assistedinject',
     '//lib/guice:guice-servlet',
     '//lib/jgit:jgit',
+    '//lib/joda:joda-time',
     '//lib/log:api',
     '//lib/mina:sshd',
   ],