Add missing dependency on Jackson2 library

The com.google.client.api.json.jackson2 package depends on the com.fasterxml.jackson.core library, but this was not properly reflected in the Nomulus build files. The problem was masked because the library was getting pulled in by other dependencies, but when they were removed, the bug came out.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=142291622

java/com/google/api/client/json/jackson2/BUILD

@@ -6,4 +6,7 @@ java_library(
         "@google_api_client//jar",
         "@google_http_client_jackson2//jar",
     ],
+    runtime_deps = [
+        "@jackson_core//jar",
+    ],
 )