Add annotation processor for @AutoOneOf

The dependencies we use allow to use @AutoOneOf but we forgot to include
the corresponding annotation processor. This hadn't been noticed so far
as we didn't use @AutoOneOf yet. We do want to use it for the follow-up
change I4bf204079, though.

Change-Id: I720d0d20f990de5c25958b651e26ee46770fa82f
diff --git a/lib/auto/BUILD b/lib/auto/BUILD
index b60a101..1da7f50 100644
--- a/lib/auto/BUILD
+++ b/lib/auto/BUILD
@@ -18,12 +18,22 @@
     ],
 )
 
+java_plugin(
+    name = "auto-oneof-plugin",
+    processor_class = "com.google.auto.value.processor.AutoOneOfProcessor",
+    deps = [
+        "@auto-value-annotations//jar",
+        "@auto-value//jar",
+    ],
+)
+
 java_library(
     name = "auto-value",
     data = ["//lib:LICENSE-Apache2.0"],
     exported_plugins = [
         ":auto-annotation-plugin",
         ":auto-value-plugin",
+        ":auto-oneof-plugin",
     ],
     visibility = ["//visibility:public"],
     exports = ["@auto-value//jar"],
@@ -35,6 +45,7 @@
     exported_plugins = [
         ":auto-annotation-plugin",
         ":auto-value-plugin",
+        ":auto-oneof-plugin",
     ],
     visibility = ["//visibility:public"],
     exports = ["@auto-value-annotations//jar"],