Add missing cstring include in gl_enum_utils. Bug: angleproject:7752 Change-Id: I44ac0996673065b9633b35e4b07c5150a1ae3dcd Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3970767 Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>