Use defvar for rust-top-item-beg-re instead

Since rust-top-item-beg-re hasn't been defined yet, using defvar instead of
setq is more appropriate here (and also silences compilation warnings).
auto merge of #13922 : Rufflewind/rust/patch-1, r=pnkfelix

Since rust-top-item-beg-re hasn't been defined yet, using defvar instead of
setq is more appropriate here (and also silences compilation warnings).