Missing .gitmodules after addition of octane submodule

[ederlf]

1. What did you do? If possible, provide a simple script for reproducing the error.

Octane was added as a submodule, but the .gitmodules file was not included. 
It will fail if we clone and try to update the git submodules:

git submodule update --init                               

I could add it manually, but for automatic builds where we run `git submodule update --init` it would be nice to have the modules in place.

2. What did you expect to see?

git submodule update --init  should not fail and output something like below:

Submodule 'ci/octane' (https://github.com/laravel/octane.git) registered for path 'ci/octane'
Cloning into '/tmp/ext-openswoole/ci/octane'...
Submodule path 'ci/octane': checked out '7ba4282697dd4f3d58704139cf02a5dfaad4e86c'

3. What did you see instead?

fatal: No url found for submodule path 'ci/octane' in .gitmodules 

4. What version of OpenSwoole are you using (show your php --ri openswoole)?

v22.1.0

5. What is your machine environment used (show your uname -a & php -v & gcc -v) ?

Not really relevant.

You can also try the following OpenSwoole support channels:

• Documentation - Documentation for Open Swoole
• Slack - Slack channel of Open Swoole
• Discord - Discord server of Open Swoole