IECoreScene build failure on architectures with unsigned char type #1412

Description

Building IECoreScene on an architecture with an unsigned char type results in a warning-as-errors compile failure for:

(sorry I don't have the log output to hand just now, will re-build when I get a chance)

Steps to Reproduce

1. Build cortex under Alma 9 (gcc 11) on aarch64. (Apple M2)