Update base for Update on "Reuse GELU implementation from PyTorch core"

kernels/optimized doesn't need to support embedded systems, so it can just take a header-only dep on PyTorch.

Note that, because we will pick up Sleef internally and ignore it
externally thanks to ATen vec, this PR gets to enable optimized GELU in OSS.

Testing: CI to make sure this doesn't break mobile build modes; happy to take advice on anything not currently covered that might break.

Differential Revision: [D66335522](https://our.internmc.facebook.com/intern/diff/D66335522/)

[ghstack-poisoned]

Author: swolchok

shim/xplat/executorch/build/env_interface.bzl

@@ -140,7 +140,8 @@ def _patch_headers(kwargs):
 
     # header_namespace is to workaround the fact that all C++ source files are having the pattern:
     # `include <executorch/.../*.h>` but BUCK2 root is at executorch/ so the `executorch/` prefix is redundant.
-    kwargs["header_namespace"] = "executorch/" + native.package_name()
+    if "header_namespace" not in kwargs:
+        kwargs["header_namespace"] = "executorch/" + native.package_name()
     return kwargs
 
 def _patch_pp_flags(kwargs):