Corrects compile error on ruby 2.0

src/ruby/ext/grpc/rb_call.c

src/ruby/ext/grpc/rb_channel.c

src/ruby/ext/grpc/rb_channel_args.c

src/ruby/ext/grpc/rb_completion_queue.c

src/ruby/ext/grpc/rb_credentials.c

src/ruby/ext/grpc/rb_grpc.c

src/ruby/ext/grpc/rb_server.c

src/ruby/ext/grpc/rb_server_credentials.c