Fix build error for `make test-unit`
This fix tries to address the issue raised by #33856 where
`make test-unit` will result in the failure:
```
...
...
dockerversion/useragent.go:20: undefined: Version
dockerversion/useragent.go:22: undefined: GitCommit
ok github.com/docker/docker/api 0.026s coverage: 68.0% of statements
ok github.com/docker/docker/api/errors 0.003s coverage: 100.0% of statements
FAIL github.com/docker/docker/api/server [build failed]
make: *** [test-unit] Error 2
```
The issue is because in case of `make test-unit`, `source "${MAKEDIR}/.go-autogen"`
is missing.
This caused the `make test-unit` failure.
This fix adds `source "${MAKEDIR}/.go-autogen"` in `hack/make/test-unit`
This fix fixes #33856.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>