Fix ChildProcess::Unix::Process#wait method with leader processes

Issue #151 reported a problem where in the update from Ruby 2.5 to Ruby
2.6 some programs would block forever on a `Process.waitpid2` call.

Changing the call to pass in a positive PID instead of the negative PID
fixes the issue as it will call the `waitpid` system call right away
instead of delaying it for later.

See #151 for further discussion of the possible reasons for the
behavior.

lib/childprocess/unix/process.rb

@@ -50,7 +50,8 @@ def wait
         if exited?
           exit_code
         else
-          _, status = ::Process.waitpid2 _pid
+          _, status = ::Process.waitpid2(@pid)
+
           set_exit_code(status)
         end
       end