rpc: Make rpcgen produce ANSI C code

rpc: Make rpcgen produce ANSI C code

This is the default for the version of rpcgen shipped with
Linux distributions, but the one in macOS and possibly others
default to K&R C, which modern compilers don't appreciate.

Luckily, all versions of rpcgen shipped with our target
platforms seem to support the -C option.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Laine Stump <laine@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>