libavb: Fix a warning with clang-15

With clang-15 we now will get warnings such as:

warning: a function declaration without a prototype is deprecated in all
versions of C [-Wstrict-prototypes]

And it is easy enough to address this warning here, as we aren't
concerned with re-syncing with an upstream.

Signed-off-by: Tom Rini <trini@konsulko.com>
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Cc: Igor Opaniuk <igor.opaniuk@gmail.com>