Buildsystem: Include icu4c into ft_linked_deps check

[j20001970]

Closes #91401

Building freetype, libpng, zlib, graphite, harfbuzz as system-provided while leaving icu4c as built-in leads to editor crash when loading GDExtension libraries that uses std::call_once in static initialization. By including builtin_icu4c into ft_linked_deps check, these libraries as a whole, will be either built-in or system-provided.

[j20001970]

There is a known bug (#100301) that breaks writing to stdout from GDExtension, it will affect Godot builds that use icu4c as system-provided if this get merged.

Closing this in favor of #100052