[BE] Fix CMake dev warning (#81580)
Remove spurious comma to fix
```
CMake Warning (dev) at aten/src/ATen/CMakeLists.txt:124:
Syntax Warning in cmake code at column 79
Argument not separated from preceding token by whitespace.
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81580
Approved by: https://github.com/osalpekar, https://github.com/weiwangmeta
