greybus: interface: declare gb_interface_destroy()

Add a public declaration for gb_interface_destroy(), matching
gb_interface_create().

It's not yet used outside "interface.c" but I suppose it
could be, and its scope is currently public.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>