llext: add dependencies

When an LLEXT object uses symbols of another such object, it depends
on it. Such dependencies have to be tracked to prevent their
accidental unloading. Ideally we should be able to track arbitrary
numbers of such dependencies, but this is a bit difficult. In this
first implementation we use a fixed-size array, currently consisting
of 8 entries.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>