platform: goldfish: pipe: Make symbol 'goldfish_pipe_dev' static

Fixes the following sparse warning:

drivers/platform/goldfish/goldfish_pipe.c:214:26: warning:
 symbol 'goldfish_pipe_dev' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>