Add common/ to .gitignore

BUG=skia:2682
NOTRY=true
R=rmistry@google.com

Author: borenet@google.com

Review URL: https://codereview.chromium.org/342353002
diff --git a/.gitignore b/.gitignore
index ad0f8ff..27ddb22 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,7 @@
 .gclient*
 .gm-actuals
 TAGS
+common
 gyp/build
 out
 platform_tools/chromeos/third_party/externals