Do not use multiprocessing.Pool when --processes=1.

Purpose of this change is not to save some execution time,
but to avoid forking another process. In other applications
that use a wrapper to call this python script, it is difficult
to get overwritten file I/O functions work in a subprocess.
So the wrapper will call warn.py with --processes=1.

Test: run "warn.py --processes=1 build.log"
Change-Id: I5998d5c70d81a456c86eb4002f444a4a60135477