Include jackson-annotation explicitly

jackson-annotation is not included anymore as part of Gerrit from
v3.5 onwards, therefore the plugin needs to explicitily
require its version and include it in the plugin's jar.

Bug: Issue 15546
Change-Id: I7c5055ad34f5bd73dab53aa1940774626222306c

external_plugin_deps.bzl

diff --git a/external_plugin_deps.bzl b/external_plugin_deps.bzl
index aed23bd..e3a41e8 100644
--- a/external_plugin_deps.bzl
+++ b/external_plugin_deps.bzl
@@ -66,6 +66,12 @@
     )
 
     maven_jar(
+        name = "jackson-annotations",
+        artifact = "com.fasterxml.jackson.core:jackson-annotations:" + JACKSON_VER,
+        sha1 = "6ae6028aff033f194c9710ad87c224ccaadeed6c",
+    )
+
+    maven_jar(
         name = "joda-time",
         artifact = "joda-time:joda-time:2.10.10",
         sha1 = "29e8126e31f41e5c12b9fe3a7eb02e704c47d70b",