install headers from runtime/backend

[swolchok]

Looks like these are all public and we simply missed installing them.

[swolchok]

Stack from ghstack (oldest at bottom):

• Delete tools/cmake/{buck_util,resolve_buck}.py #13588
• Switch from print_exported_headers to CMake's installed headers in scripts/build_apple_frameworks.sh #13559
• -> install headers from runtime/backend #13558
• Add check_for_installed_private_headers_in_cmake_out #13485

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13558

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❌ 2 New Failures, 3 Cancelled Jobs

As of commit 2b5e2b6 with merge base 335de46 ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 255

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• Lint / link-check / lint-urls / linux-job (gh)
• Lint / link-check / lint-xrefs / linux-job (gh)
• Lint / lintrunner / linux-job (gh)

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[JacobSzwejbka]

why do we need one for basically every runtime subdir as opposed to just one runtime/?

[swolchok]

I wanted to keep the exclude patterns narrowly scoped.

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