build : use umbrella Headers directory for XCFramework module map (#23974)

build : use umbrella Headers directory for XCFramework module map (#23974)

The XCFramework generated by build-xcframework.sh creates a module map
that manually lists public headers.

That list can fall out of sync with the framework's Headers directory.
The module map is currently missing ggml-opt.h, which is present in the
framework headers. This can cause downstream Apple builds to fail with:

    Include of non-modular header inside framework module 'llama'

Use the framework's Headers directory itself as the module map umbrella
instead of maintaining a manual header list. This makes all public headers
under the generated framework's Headers directory part of the llama module.