graphics/openexr: upgrade to 3.1.7

This is mostly to incorporate build fixes for ARMv7 regressions,
which our 3.1.6_1 port had already brought, and bumps the library's minor
version, so let's take it although it should be functionally the same.

ChangeLog:
https://github.com/AcademySoftwareFoundation/openexr/blob/v3.1.7/CHANGES.md#version-317-march-28-2023