Add imgui.ini to .gitignore

This file is auto-generated by viewer's GUI library (to remember size and
position of windows).

BUG=skia:

Change-Id: Id363301d71d3df1d665c4e495a0d5062cdbc472b
Reviewed-on: https://skia-review.googlesource.com/9095
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/.gitignore b/.gitignore
index b75dfff..1dd8e0d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -15,6 +15,7 @@
 TAGS
 bower_components
 common
+imgui.ini
 infra/bots/tools/luci-go/linux64/cipd
 infra/bots/tools/luci-go/linux64/isolate
 infra/bots/tools/luci-go/mac64/cipd