[llvm] [gn] use `sources` instead of `inputs` for libc++ header copy action (PR #206263)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `llvm` at step 6 "test-build-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/47875

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-flang) failure: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
...
PASS: Flang :: Driver/fveclib.f90 (4634 of 4644)
PASS: Flang :: Driver/mrecip.f90 (4635 of 4644)
PASS: Flang :: Driver/nostdlib.f90 (4636 of 4644)
PASS: Flang :: Driver/mcmodel.f90 (4637 of 4644)
PASS: Flang :: Driver/options-loongarch.f90 (4638 of 4644)
PASS: Flang :: Lower/OpenMP/declare-mapper.f90 (4639 of 4644)
PASS: Flang :: Driver/fopenmp.f90 (4640 of 4644)
PASS: Flang :: Transforms/vscale-attr.fir (4641 of 4644)
PASS: Flang :: Driver/linker-options.f90 (4642 of 4644)
PASS: Flang :: Driver/omp-driver-offload.f90 (4643 of 4644)
command timed out: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2346.017762

```

</details>

https://github.com/llvm/llvm-project/pull/206263