Include string.h in //remoting/c/a/fake_async_audio_data_supplier.cc

The code in this file uses memset function, but does not include the
header file declaring this type. This leads to build failure on Linux
systems using recent glibc versions.

Bug: 1074286
Change-Id: I6bb0b1ed1cd70ef872a9809a6fff1d78eeb8de3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2206637
Reviewed-by: Lambros Lambrou <lambroslambrou@chromium.org>
Reviewed-by: Gary Kacmarcik <garykac@chromium.org>
Commit-Queue: Piotr Tworek <ptworek@vewd.com>
Cr-Commit-Position: refs/heads/master@{#770015}