um: signal: move pid variable where needed

We have W=1 warnings on 64-bit because the pid is only
used in branches on 32-bit; move it inside to get rid of
the warnings.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>