Make bgplob an exception to dangling rule check.

It's built explicitly by Microfactory but is under out/ and as such, it
would be normally flagged as a file in out/ that doesn't have a
generating rule.

Previously, a no-op rule was added to the Ninja file to prevent this
from happening, but there is no good reason to have two mechanisms to
exempt files under out/ from that check.

Test: Presubmits.
Change-Id: I37fc458ecc5f5734cd81bce2f4894063658df38f