Plugin API: Export log4j library

Change-Id: I6f878e4f31ebe6fb1e32998b784892746044e9c9
diff --git a/gerrit-plugin-api/BUCK b/gerrit-plugin-api/BUCK
index 7e38726..cd688a9 100644
--- a/gerrit-plugin-api/BUCK
+++ b/gerrit-plugin-api/BUCK
@@ -61,6 +61,7 @@
     '//lib/jgit/org.eclipse.jgit.http.server:jgit-servlet',
     '//lib/joda:joda-time',
     '//lib/log:api',
+    '//lib/log:log4j',
     '//lib/mina:sshd',
     '//lib/ow2:ow2-asm',
     '//lib/ow2:ow2-asm-analysis',