Add missing #include and namespace to nbla/cuda/version.hpp

[Tosainu]

This pull request fixes the compile errors when using nbla/cuda/version.hpp.

$ g++ --version
g++ (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat main.cc
#include <iostream>
#include <nbla/cuda/version.hpp>

auto main() -> int {
  std::cout << nbla::nbla_ext_cuda_version() << std::endl;
}
$ g++ main.cc -lnnabla_cuda
In file included from main.cc:2:0:
/usr/local/include/nbla/cuda/version.hpp:18:7: error: 'string' does not name a type; did you mean 'stdin'?
 const string nbla_ext_cuda_version(void);
       ^~~~~~
       stdin
/usr/local/include/nbla/cuda/version.hpp:20:7: error: 'string' does not name a type; did you mean 'stdin'?
 const string nbla_ext_cuda_cuda_version(void);
       ^~~~~~
       stdin
/usr/local/include/nbla/cuda/version.hpp:21:7: error: 'string' does not name a type; did you mean 'stdin'?
 const string nbla_ext_cuda_cudnn_version(void);
       ^~~~~~
       stdin
/usr/local/include/nbla/cuda/version.hpp:23:7: error: 'string' does not name a type; did you mean 'stdin'?
 const string nbla_ext_cuda_author(void);
       ^~~~~~
       stdin
/usr/local/include/nbla/cuda/version.hpp:24:7: error: 'string' does not name a type; did you mean 'stdin'?
 const string nbla_ext_cuda_author_email(void);
       ^~~~~~
       stdin
/usr/local/include/nbla/cuda/version.hpp:25:7: error: 'string' does not name a type; did you mean 'stdin'?
 const string nbla_ext_cuda_build_number(void);
       ^~~~~~
       stdin
main.cc: In function 'int main()':
main.cc:5:16: error: 'nbla' has not been declared
   std::cout << nbla::nbla_ext_cuda_version() << std::endl;
                ^~~~