buildman: Allow lines without a symbol

The 'nm' tool can produce lines without a symbol, for example:

   00000004 t

Silently skip these and anything else without three fields. Drop the
warning since there is nothing the user can do about it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Tom Rini <trini@konsulko.com>