.gitignore: add .gclient_previous_custom_vars

Bug: 1609
Change-Id: Iec1ffedadb85b8f8ca80440b9e7fd7ef437df5bf
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/113120
Kokoro: Ben Clayton <bclayton@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@chromium.org>
diff --git a/.gitignore b/.gitignore
index bb0834e..563775b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -4,6 +4,7 @@
 /.cipd
 /.gclient
 /.gclient_entries
+/.gclient_previous_custom_vars
 /.gclient_previous_sync_commits
 /build
 /buildtools