sys_util: shm: Make clippy clean
sys_util: shm: Make clippy clean

clippy wants a default impl for `MemfdSeals`

Signed-off-by: Dylan Reid <c791bfe44996c1eed9c7149a0c5f12a86d58eb85@chromium.org>
Change-Id: I3202037f7b82d7d4e63154a349c505fd7707bb9a
Reviewed-on: https://chromium-review.googlesource.com/1510070
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>

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