build: suppress warnings in fmt when building on mac

[antond-weta]

Description

This suppresses the warnings in the fmt library code which have started appearing recently when building on a Mac.
I suspect the warnings were triggered by the clang toolchain being updated to v21, but to be fair I haven't tried to downgrade to confirm that.

Tests

No extra tests. The changes allow OIIO to build with "exit on warning" on Mac with the latest build tools, whereas without the changes the build fails.

There are some runners failing on CI, but that seems unrelated.

Checklist:

• I have read the guidelines on contributions and code review procedures.
• I have read the Policy on AI Coding Assistants
  and if I used AI coding assistants, I have an Assisted-by: TOOL / MODEL
  line in the pull request description above.
• I have updated the documentation if my PR adds features or changes
  behavior.
• I am sure that this PR's changes are tested in the testsuite.
• I have run and passed the testsuite in CI before submitting the
  PR, by pushing the changes to my fork and seeing that the automated CI
  passed there. (Exceptions: If most tests pass and you can't figure out why
  the remaining ones fail, it's ok to submit the PR and ask for help. Or if
  any failures seem entirely unrelated to your change; sometimes things break
  on the GitHub runners.)
• My code follows the prevailing code style of this project and I
  fixed any problems reported by the clang-format CI test.
• If I added or modified a public C++ API call, I have also amended the
  corresponding Python bindings. If altering ImageBufAlgo functions, I also
  exposed the new functionality as oiiotool options.