Fix signed/unsigned comparison warning

[jlowe]

After #966 builds with full warnings enabled as errors could fail with the following error:

/miniconda3/envs/cudf_dev/include/rmm/device_buffer.hpp: In member function ‘int64_t rmm::device_buffer::ssize() const’:
/home/jlowe/miniconda3/envs/cudf_dev/include/rmm/device_buffer.hpp:327:19: error: comparison of integer expressions of different signedness: ‘std::size_t’ {aka ‘long unsigned int’} and ‘long int’ [-Werror=sign-compare]
  327 |     assert(size() < std::numeric_limits<int64_t>::max() && "Size overflows signed integer");
      |            ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

This fixes the warning by casting the numerical limit to std::size_t before comparing.

[vyasr]

@gpucibot merge

[harrism]

Ah, sorry. This would only have been a debug build failure, which indeed I didn't test. Thanks @jlowe for fixing.