Fix compilation error (cannot find value leader in this scope)

[arthurdandrea]

Hi, updating watchexec I've got the following error, seemed easy so I'm sending the PR without a bug report.

error[E0425]: cannot find value `leader` in this scope
  --> $HOME/.cargo/registry/src/index.crates.io-6f17d22bba15001f/process-wrap-8.0.2/src/tokio/process_group.rs:99:26
   |
99 |                 setpgid(Pid::this(), leader)
   |                                      ^^^^^^
   |
help: you might have meant to use the available field
   |
99 |                 setpgid(Pid::this(), self.leader)
   |                                      +++++

PS: Thanks for the awesome libs.

[passcod]

i'm currently without a computer so i'll be a while before i can release, sorry