rust: task: make Pid type alias public

rust: task: make Pid type alias public

The Pid type alias represents the integer type used for pids in the
kernel. It's the Rust equivalent to pid_t, and there are various methods
on Task that use Pid as the return type.

Binder needs to use Pid as the type for function arguments and struct
fields in many places. Thus, make the type public so that Binder can
access it.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Link: https://lore.kernel.org/r/20250130-task-pid-pub-v1-1-508808bcfcdc@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>