Add -Werror=-Wno-error=incomplete-record-selectors to hadrian-multi

In the main MR, -Wall now includes -Wincomplete-record-selectors.
However `hadrian-multi` has many, many warnings about incomplete
record selectors, so this patch stops those warnings being treated
as errors.  (See discussion on !13308.)

A better fix would be to remove the use of incomplete record
selectors, since each of them represents a potential crash.