Explicitly include <cstdlib> in buffer.cc

After https://github.com/llvm/llvm-project/pull/139586, this header is
no longer transitively included by <cmath>, so we need to include it
directly.

Bug: 427774670
Change-Id: I9d892d30f410ee55d55afddf33c7aabc3be21344
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6679831
Commit-Queue: Mohannad Farrag <aymanm@google.com>
Auto-Submit: Devon Loehr <dloehr@google.com>
Reviewed-by: Mohannad Farrag <aymanm@google.com>
Cr-Commit-Position: refs/heads/main@{#1479187}