Adding third_party/cygwin to git ignore

Review URL: http://webrtc-codereview.appspot.com/280003

git-svn-id: http://webrtc.googlecode.com/svn/trunk@963 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/.gitignore b/.gitignore
index 74c5aaf..48d2fd1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -36,6 +36,7 @@
 /src/supplement.gypi
 /testFiles
 /testing
+/third_party/cygwin
 /third_party/expat
 /third_party/google-gflags/src
 /third_party/jsoncpp