Add missing string.h include in //third_party/zlib/google.

The code in zip_internal.cc uses memcpy but fails to include header file
defining this function. This leads to build failures on Linux systems
using glibc newer than 2.29.

Bug: 1074286
Change-Id: I58e520994485923b606396035cae79f0291b79f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2172785


Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Piotr Tworek <ptworek@vewd.com>
Cr-Original-Commit-Position: refs/heads/master@{#764158}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: cfb0de6d748ebab7d382568eef88efacde44aee2