Build: MPMD w/ MPI Only

[ax3l]

Fix no-MPI build logic.

Seen for no-MPI builds in https://github.com/conda-forge/pyamrex-feedstock/pull/19

[ 60%] Building CXX object CMakeFiles/pyAMReX_2d.dir/src/Base/MPMD.cpp.o
$BUILD_PREFIX/bin/x86_64-conda-linux-gnu-c++ -DAMREX_SPACEDIM=2 -DpyAMReX_2d_EXPORTS -I$SRC_DIR/src -I$PREFIX/share/amrex/C_scripts -isystem $PREFIX/include/python3.10 -fvisibility-inlines-hidden -fmessage-length=0 -march=nocona -mtune=haswell -ftree-vectorize -fPIC -fstack-protector-strong -fno-plt -O2 -ffunction-sections -pipe -isystem $PREFIX/include -fdebug-prefix-map=$SRC_DIR=/usr/local/src/conda/pyamrex-24.05 -fdebug-prefix-map=$PREFIX=/usr/local/src/conda-prefix -Wall -Wextra -Wpedantic -Wshadow -Woverloaded-virtual -Wunreachable-code -O3 -DNDEBUG -std=c++17 -fPIC -fvisibility=hidden -pthread -fopenmp -flto -fno-fat-lto-objects -MD -MT CMakeFiles/pyAMReX_2d.dir/src/Base/MPMD.cpp.o -MF CMakeFiles/pyAMReX_2d.dir/src/Base/MPMD.cpp.o.d -o CMakeFiles/pyAMReX_2d.dir/src/Base/MPMD.cpp.o -c $SRC_DIR/src/Base/MPMD.cpp
/home/conda/feedstock_root/build_artifacts/pyamrex_1715553474293/work/src/Base/MPMD.cpp:11:10: fatal error: AMReX_MPMD.H: No such file or directory
   11 | #include <AMReX_MPMD.H>
      |          ^~~~~~~~~~~~~~

Follow-up to #271

cc @siddanib