Fix exp_short_init_compact_seq() signature

[DavisVaughan]

Goes with r-lib/slider#224, but doesn't require it

The actual implementation of this was right

void exp_short_init_compact_seq(int* p, R_len_t start, R_len_t size, bool increasing) {
  init_compact_seq(p, start, size, increasing);
}

but the signature here in vctrs was wrong. And then we copied over that wrong signature into slider as well.

Luckily because it returns void and we never look at the return value, it was fine. But CRAN picked up on it in one of their checks awhile back.