rutabaga_gfx: use as to perform type conversion
rutabaga_gfx: use as to perform type conversion

On Windows os_handle is of type void*, can't use Into::into to cast to
i64. Use as i64 instead.

Test: launch_cvd
Change-Id: I7fe44dbadf5214ff9bfd81482f5b598091a8f692
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3448217
Reviewed-by: Gurchetan Singh <99434bba058e25b6203b8ae055a4f697dad0ef64@chromium.org>
Reviewed-by: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>
Auto-Submit: Kaiyi Li <f6fa95ef69f95b15c9d6b9a996e5e7cbcfb76e84@google.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Kaiyi Li <f6fa95ef69f95b15c9d6b9a996e5e7cbcfb76e84@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm