This prevents MSVC complaining with
warning C4201: nonstandard extension used: nameless struct/union
The struct might seem unnecessary to the occasional reader (it did seem
so to this commit's author), but it is not! It is required to align the
fields to a boundary, which is verified by the test suite. Removing that
"unnecessary" `struct` results in this failure:
1: Test command: mimalloc-test-api
[...]
1: test: malloc-zero... mimalloc: assertion failed: at src/page.c:591, mi_page_init
1: assertion: "!mi_page_has_aligned(page)"
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
559688ec64