cmake: Fix variable typo for USE_SYSTEM_PYBIND11. (#80272)
The correct variable name should be USE_SYSTEM_PYBIND11, as defined in
the root CMakeLists.txt. In cmake/Dependencies.cmake, it is incorrectly
written as USE_SYSTEM_BIND11, but cmake will not complain about this.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80272
Approved by: https://github.com/suo
