Fix interface includes after directory split

[whahnsr]

Proposed Changes

Correct three relative include paths affected by the deeper cfd and fsi
interface directory structure.

Without these changes, a fresh serial build of the current develop branch
fails because the compiler cannot locate C2DContainer.hpp and
CInterpolator.hpp.

Related Work

Found while preparing and profiling a fresh build of the current develop
branch.

Generative AI usage

OpenAI Codex assisted with diagnosis, validation, and preparation of this
change.

Co-authored-by: OpenAI Codex noreply@openai.com

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary. Not necessary; this repairs compilation.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary. Not necessary.

Test Results

Configured with Meson at warning level 3 and built and installed successfully
with GCC 13.3.0 after the include paths were corrected.