Add additional gitignore pattern for build directories.
Add additional gitignore pattern for build directories.

Related to #9

google/iree,iree-org/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,iree-org/iree,google/iree,google/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,google/iree