Add missing cstring include to Uniform.cpp.
Add missing cstring include to Uniform.cpp.

This file was missing on GCC builds.

BUG=angle:773

Change-Id: I6222b5ee563d64967d878c422abd464c8c92e646
Reviewed-on: https://chromium-review.googlesource.com/231855
Tested-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Brandon Jones <303ddc8529a3012e7fb187d611f08537af0cc183@chromium.org>

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