[gperftools] Fix volatile usage for C++20

TEST=no behavior change

Change-Id: Id66a4107cf484b9dbba0f514fdfd693b17135c4e
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/github.com/gperftools/gperftools/+/635743
Reviewed-by: Roland McGrath <mcgrathr@google.com>