[google] Fix get_bucket_acl request method in Google Cloud Storage

This PR fixes `get_bucket_acl` method visibility :
Before:
```ruby
require './lib/fog/google' 
storage = Fog::Storage::Google.new(...) 
storage.methods.grep(/^get_b/)       # => [:get_bucket]
```
After:
```ruby
require './lib/fog/google' 
storage = Fog::Storage::Google.new(...) 
storage.methods.grep(/^get_b/)       # => [:get_bucket, :get_bucket_acl]
```