Add missing LTC headers, re-enble xla configuration

Addresses XLA test failures due to missing PyTorch lazy tensor backend headers:
```
“fatal error: ‘torch/csrc/lazy/backend/backend_device.h’ file not found” from pytorch-xla-linux-bionic-py3.7-clang8
```

Pull Request resolved: pytorch#74756
Approved by: https://github.com/seemethere

.github/workflows/pull.yml

@@ -212,7 +212,6 @@ jobs:
       build-generates-artifacts: false
 
   pytorch-xla-linux-bionic-py3_7-clang8-build:
-    if: ${{ false }}
     name: pytorch-xla-linux-bionic-py3.7-clang8
     uses: pytorch/pytorch/.github/workflows/_linux-build.yml@master
     with:

setup.py

@@ -1051,6 +1051,7 @@ def print_box(msg):
                 'include/torch/csrc/profiler/*.h',
                 'include/torch/csrc/utils/*.h',
                 'include/torch/csrc/tensor/*.h',
+                'include/torch/csrc/lazy/backend/*.h',
                 'include/torch/csrc/lazy/core/*.h',
                 'include/pybind11/*.h',
                 'include/pybind11/detail/*.h',