Suppress DPC++/SYCL header deprecation warnings when building dpnp.tensor on Windows (#2993)

Re-land of #2984. That PR was a stacked PR targeting the intermediate
branch `pin-conda-to-avoid-installing-faulty-build`, whose own PR
(#2982) was **closed, not merged**. As a result, #2984 was merged into
that dead intermediate branch and its changes never reached `master`.
GitHub won't allow reopening a merged PR, so this re-lands the identical
change directly against `master`.

This is a follow-up to #2856 and #2770. The `dpnp.tensor` target never
got the same `SYSTEM` treatment, so it was the one target still leaking
these warnings.

The Windows conda build produced a flood of `-Wdeprecated-declarations`
warnings (hundreds of thousands of lines in the CI log). All of them
originate inside the DPC++/SYCL headers themselves
(`.../Library/include/sycl/...`) — none come from dpnp's own sources —
so they are out of dpnp's control and are pure noise. They came
exclusively from the `dpnp.tensor` `_tensor_*_impl` pybind11 modules
built in `dpnp/tensor/CMakeLists.txt`.

In the `dpnp/tensor` target loop, split the include directories so
dpnp's own headers stay `PRIVATE`, and the third-party headers
(`SYCL_INCLUDE_DIR`, `Dpctl_INCLUDE_DIR`, generated Cython headers) are
added as `SYSTEM PRIVATE`, mirroring the pattern established in #2770
for the `backend/extensions/*` targets. 6671952

Author: github-actions[bot]

reference/comparison.html

@@ -1040,7 +1040,7 @@
 <h1>Comparison Table NumPy/ DPNP/ CuPy<a class="headerlink" href="#comparison-table-numpy-dpnp-cupy" title="Link to this heading">¶</a></h1>
 <p>Here is a list of NumPy and CuPy APIs and its corresponding DPNP implementations.</p>
 <p><code class="docutils literal notranslate"><span class="pre">-</span></code> in DPNP column means that DPNP implementation is not provided yet.</p>
-<p><strong>NumPy(v2.5.1) / DPNP(v0.21.0dev2+18.g3a9d73dba80) / CuPy(v14.1.1) APIs</strong></p>
+<p><strong>NumPy(v2.5.1) / DPNP(v0.21.0dev2+19.g667195234a8) / CuPy(v14.1.1) APIs</strong></p>
 <section id="summary">
 <h2>Summary<a class="headerlink" href="#summary" title="Link to this heading">¶</a></h2>
 <div class="table-wrapper docutils container">