Add gerrit-package-plugins folder to .gitignore

This folder appears as an untracked item after building the
verify target in maven.

Change-Id: Iae15a649c3acfff577ffd3edae171e6e84060e74
diff --git a/.gitignore b/.gitignore
index 26c4a22..5c55d5e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,7 @@
 /.idea
 /gerrit-parent.iml
 *.sublime-*
+/gerrit-package-plugins
 /buck-cache
 /buck-out
 /local.properties