flashmap: Fix warnings with gcc 10 BUG=b:229038573 TEST=ran unittests Change-Id: I3a3c428c4c2a9eba6d1ff353e712f1220d5728cf Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/flashmap/+/4601333 Reviewed-by: Jack Rosenthal <jrosenth@chromium.org> Tested-by: Dmitry Bilunov <kmeaw@google.com> Commit-Queue: Dmitry Bilunov <kmeaw@google.com>