Build fails with -DENABLE_MPI=OFF

[dvrogozh]

With:

• 6649993

Build of oneCCL fails if -DENABLE_MPI=OFF is passed:

$ docker run -it --rm ubuntu:24.04
# apt-get install -y g++ cmake git
# git clone https://github.com/uxlfoundation/oneCCL.git && cd oneCCL
# mkdir _build && cd _build
# cmake -DENABLE_MPI=OFF ..
# make -j1
...
/opt/oneccl/src/atl/util/pm/pmi_resizable_rt/pmi_resizable_simple_internal.cpp: In member function 'virtual atl_status_t pmi_resizable_simple_internal::pmrt_kvs_get(char*, int, void*, size_t)':
/opt/oneccl/src/atl/util/pm/pmi_resizable_rt/pmi_resizable_simple_internal.cpp:268:25: error: 'ATL_MPI_ROOT_RANK_KEY' was not declared in this scope
  268 |     if (strcmp(kvs_key, ATL_MPI_ROOT_RANK_KEY) == 0) {
      |                         ^~~~~~~~~~~~~~~~~~~~~

CC: @blazej-smorawski