Reduce noise with GCC / Clang -Wunused-parameter.

With these changes, the inclusion of the .capnp.h file generated from a relatively simple capnp file (containing multiple unsigned integers, an enumerated type, Text, Data) which I made for testing purposes doesn't trigger a bunch of -Wunused-parameter warnings anymore for me. I can focus on warnings in my own code :)

I chose to cast the unused parameters to void because I know that doing so is more portable than both self-assignment (GCC / Clang -Winit-self is designed to trigger warnings about it) and of course __attribute__((unused)).