dependencies: Update the way to get C-Thread-Pool

[nitneuk]

To use frankenphp as a golib, we need to get C-Thread-Pool as a dependency. As it's not a go dependencies, we need to version it to avoid this error :

$ go build
# github.com/dunglas/frankenphp
frankenphp.c:16:10: fatal error: 'C-Thread-Pool/thpool.h' file not found

We init it using go generate.

[dunglas]

Thanks!