[CMake] Fix the Python build with -Dtensorflow_ENABLE_GRPC_SUPPORT=OFF. (#7076)

The SWIG wrapper could not resolve the server-creation function, which
is unconditionally referenced in server_lib.i, but not built when gRPC
support is disabled. This change fixes the problem by building
server_lib.cc as part of tf_core_cpu, so it can always be resolved,
even when gRPC support is disabled.

Fixes #7019.