Add avatars-external plugin to the checks pipeline

Change-Id: I9a17d74a2068f1464ac5ea84cf7db1f0575e735b

zuul.d/projects.yaml

diff --git a/zuul.d/projects.yaml b/zuul.d/projects.yaml
index 573ed8d..bf96e39 100644
--- a/zuul.d/projects.yaml
+++ b/zuul.d/projects.yaml
@@ -81,6 +81,12 @@
         - gerrit-plugin-build
 
 - project:
+    name: plugins/avatars-external
+    check:
+      jobs:
+        - gerrit-plugin-build
+
+- project:
     name: plugins/batch
     check:
       jobs: