prio-queue: fix type of `insertion_ctr`

prio-queue: fix type of `insertion_ctr`

In 62e745ced2 (prio-queue: use size_t rather than int for size,
2024-12-20), we have converted `struct prio_queue` to use `size_t` to
track the number of entries in the queue as well as the allocated size
of the underlying array. There is one more counter though, namely the
insertion counter, that is still using an `unsigned` instead of a
`size_t`. This is unlikely to ever be a problem, but it makes one wonder
why some indices use `size_t` while others use `unsigned`. Furthermore,
the mentioned commit stated the intent to also adapt these variables,
but seemingly forgot to do so.

Fix the issue by converting those counters to use `size_t`, as well.

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