reftable/block: adapt header and footer size to return a `size_t`

reftable/block: adapt header and footer size to return a `size_t`

The functions `header_size()` and `footer_size()` return a positive
integer representing the size of the header and footer, respectively,
dependent on the version of the reftable format. Similar to the
preceding commit, these functions return a signed integer though, which
is nonsensical given that there is no way for these functions to return
negative.

Adapt the functions to return a `size_t` instead to fix a couple of sign
comparison warnings.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>