Add missing import when extending FieldBoundaryConditions in MultiRegion

[navidcy]

Perhaps this is related to those weird MultiRegion-related methods appearing with Distributed? See CliMA/ClimaOcean.jl#647 (comment)

Since the method wasn't imported, the attempt below to extent it might resulted in overwriting it?

What was missing was:

import Oceananigans.BoundaryConditions: FieldBoundaryConditions

Julia v1.12 warned me for that; that's how I found it.

  1 dependency had output during precompilation:
┌ Oceananigans
│  WARNING: Constructor for type "FieldBoundaryConditions" was extended in `MultiRegion` without explicit qualification or import.
│    NOTE: Assumed "FieldBoundaryConditions" refers to `BoundaryConditions.FieldBoundaryConditions`. This behavior is deprecated and may differ in future versions.`
│    NOTE: This behavior may have differed in Julia versions prior to 1.12.
│    Hint: If you intended to create a new generic function of the same name, use `function FieldBoundaryConditions end`.
│    Hint: To silence the warning, qualify `FieldBoundaryConditions` as `BoundaryConditions.FieldBoundaryConditions` in the method signature or explicitly `import BoundaryConditions: FieldBoundaryConditions`.
└

cc @taimoorsohail

[taimoorsohail]

The error persists with your first tranche of changes @navidcy

[navidcy]

hm... I still think this PR resolves a mistake.
I'll wait for @simone-silvestri or @siddharthabishnu to confirm

[simone-silvestri]

indeed, it does solve an issue (we had that change in #4760)