Revert "Drop NB_ from GMX_CUDA_NB_SINGLE_COMPILATION_UNIT cmake define"

Revert "Drop NB_ from GMX_CUDA_NB_SINGLE_COMPILATION_UNIT cmake define"

This reverts commit 3880255b0, which was made in confusion
stemming from combination of multiple CUDA compilation units,
disabling CUDA textures, and NB CUDA module structure.
The define in question is actually NB-exclusive,
and PME with CUDA does not need to check it to declare
extern texture references. As PME textures are not accessed
from different PME kernels, those extern declarations are removed
in the child change Ie846193c71142ff5e519e990ef1155b534546a9b.

Change-Id: I75a0e62bc92c7161ba0fbf00d8db2f35cef80bc7