build: allow "building" rts-alloc on windows

[OliverNChalk]

Right now if a downstream consumer somehow gets rts-alloc in their dependency tree, then windows builds will fail. This relaxes that such that the downstream consumer has to actually use unix only elements of rts-alloc (or a crate that depends on rts-alloc) for this to break builds. This is not strictly required but is slightly more forgiving to downstream users.