Fix HDRS_LIST building when paths in header inclusion tree include spaces

[lancelotblanchard]

Proposed changes

In mlx/backend/metal/make_compiled_preamble.sh, doing declare -a HDRS_LIST=($HDRS) breaks when the header inclusion tree $HDRS has paths with spaces. This manifests by headers not being properly linked, and a bunch of grep errors during build. This PR fixes header inclusion to support paths with spaces.

Checklist

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• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)