Fix type mismatch for GetVirtualSizeWith<Descendants|Ancerstors>

Summary:
These methods are declared to return an uint64_t but there definition
returns a size_t, which breaks the build on 32 bit platforms (and OSX
for some reason).

Test Plan:

ninja check

Run the Gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5024