Add a default gen_version.go
Add a default gen_version.go

Without this in place "go get ./..." on the top level directory
will fail on a fresh checkout.

BUG=skia:

Review URL: https://codereview.chromium.org/834773003

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot