libubox: Add ability to find ordinal position inside a table

libubox: Add ability to find ordinal position inside a table

When adding anonymous objects to an array, the only way to select
those objects (as in "json_select", i.e. moving the cursor) is to
select them via their ordinality.

This function returns a selectable index to those objects.

Signed-off-by: Philip Prindeville <[email protected]>
Link: https://github.com/openwrt/libubox/pull/33
Signed-off-by: Álvaro Fernández Rojas <[email protected]>