MSVC doesn't like |const size_t len| in a function definition where the
declaration was just |size_t len| without the |const|. Also, MSVC needs
declarations of parameterless functions to have a |void| parameter list.

Change-Id: I91e01a12aca657b2ee1d653926f09cc52da2faed
Reviewed-on: https://boringssl-review.googlesource.com/4329
Reviewed-by: Adam Langley <agl@google.com>