greybus: interface: localize gb_interface_destroy()

Its not referenced by svc or any other code anymore, lets stop exposing
it to rest of the files.

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