abi: let extract_symbols print to stdout by default

If not passing --whitelist, the script is rather useless as it just
consumes some files and does not do anything with them. Hence, make it
slightly more useful by printing to stdout in that case.

Change-Id: Ia33c803a875a5ce35e0482328ed2b0b916a4a93d
Signed-off-by: Matthias Maennich <maennich@google.com>