[self_freeze] Fix sandbox

This CL is a follow-up to
https://chromium-review.googlesource.com/c/chromium/src/+/5887346, which
mistakenly did not include the correct header for MADV_PAGEOUT, leading
to the sandbox to always skip the added code.

This CL corrects this by including the required header.

Bug: 344550156
Change-Id: Ib7bfe6b1e8551c8ae16cad8287376e3633eeeb07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5974699
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Commit-Queue: Thiabaud Engelbrecht <thiabaud@google.com>
Cr-Commit-Position: refs/heads/main@{#1375349}