This fixes an issue where an animated modifier property that's used as
variable in a driver wouldn't animate that driver's value.
Building the relations for the driver target creates the relation
`PARAMETERS_EVAL` → `DRIVER(variable)`. Building the relations for the
FCurve targeting the modifier property creates the relation
`ANIMATION_EXIT` → `GEOMETRY_EVAL_INIT`.
This means that there is NOT a relation `ANIMATION_EXIT` →
`PARAMETERS_EVAL`, and as a result, the driver is not properly updated
when its variable reads animated data. This is resolved in this commit
by adding the missing relation.
Differential Revision: https://developer.blender.org/D7615
079fc65e0a