[libfmq] Prepare for rolling zerocopy

This commit adds conditional compilation which will permit a future
change to `BUILD.gn` to add a `--cfg` which will switch libfmq from
using the old zerocopy names to using the new ones.

Bug: 367420574
Change-Id: I23635d207c20effc20b21cf566725660908f3ad8
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/android.googlesource.com/platform/system/libfmq/+/1121495
Reviewed-by: Yegor Pomortsev <ypomortsev@google.com>
1 file changed