goldfish_pipe: make tasklet goldfish_interrupt_tasklet static

Tasklet goldfish_interrupt_tasklet is local to the source and
does not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'goldfish_interrupt_tasklet' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>