c generator require --autodrop-borrows option

[韩朴宇]

--autodrop-borrows don't have deafult option and become a required option in wit-bindgen cli.

> wit-bindgen c
error: the following required arguments were not provided:
  --autodrop-borrows <AUTODROP_BORROWS>
  <DOCUMENT>

Usage: wit-bindgen c --autodrop-borrows <AUTODROP_BORROWS> <DOCUMENT>

For more information, try '--help'.

[Alex Crichton]

Thanks! That was a mistake and is fixed in #858