[ntuple] libdaos_mock: remove compiler warnings in `daos.h`; see below.
[ntuple] libdaos_mock: remove compiler warnings in `daos.h`; see below.

- [-Wunused-parameter] warning in `daos_obj_generate_id()` function.

- [-Wextern-c-compat]: empty structures are not allowed by C99; GNU C extensions
and ISO C++ take it as if it had one `char` member, so that the address of
different objects is also different. Therefore, a member of type `char` has
been added so that the struct layout is the same in C and C++.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root