Fix GN build for libGLESv2.dll. In a previous fix to a Clang build error, we had neglected to update the GN build. Broken in commit 07d49ef535. BUG=angle:707 Change-Id: Ic11aa907d591540e915bec60f3bafaea471689d5 Reviewed-on: https://chromium-review.googlesource.com/210652 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Brett Wilson <brettw@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>