TBR: third_party: Add '/.jiri' to .gitignore in all repos.

Change-Id: I53fda893f568659320f3a462e1778cd342e2ab14
MultiPart: 27/28
PresubmitTest: none
diff --git a/.gitignore b/.gitignore
index e8525b5..41f44a4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,3 +1,5 @@
+/.jiri
+#TODO(nlacasse): Get rid of .v23 below once v23->jiri transition is complete.
 /.v23
 
 # Built files from third-party C libraries