CI: build the FFT example for the size diff workflow.

Author: microbit-carlos

.github/workflows/size-diff.yml

@@ -72,6 +72,10 @@ jobs:
           EOF
           echo "coda.json after:"
           cat codal.json
+      - name: Move fft example
+        run: |
+          mv libraries/codal-microbit-v2/samples/fft_example.cpp source/main.cpp
+          cat source/main.cpp
       - name: Build using build.py
         run: python build.py
       - name: Save ELF file in a different directory