Make some global variables static

src/ruby/ext/grpc/rb_call.c

src/ruby/ext/grpc/rb_call.h

src/ruby/ext/grpc/rb_channel.c

src/ruby/ext/grpc/rb_channel.h

src/ruby/ext/grpc/rb_completion_queue.c

src/ruby/ext/grpc/rb_completion_queue.h

src/ruby/ext/grpc/rb_credentials.c

src/ruby/ext/grpc/rb_credentials.h

src/ruby/ext/grpc/rb_grpc.c

src/ruby/ext/grpc/rb_grpc.h

src/ruby/ext/grpc/rb_server.c

src/ruby/ext/grpc/rb_server.h

src/ruby/ext/grpc/rb_server_credentials.c

src/ruby/ext/grpc/rb_server_credentials.h