IWYU: add cstring for memset/memcpy in h264_parser

Bug: None
Change-Id: Iad051c74f2738c891f79219621cb4e10930f95d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3065658


Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Stephan Hartmann <stha09@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#908592}