greybus: interface: make gb_interface_destroy() static

The function is only called locally, so mark it static to make sparse
happy.

Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>