ci: install fftw3 before building

.github/workflows/python_skbuild_wheels.yml

@@ -1,4 +1,4 @@
-name: Build
+name: Build Python wheels (skbuild)
 
 on: [push, pull_request]
 
@@ -13,6 +13,10 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Install fftw
+        run: |
+          sudo apt-get install -y libfftw3-dev
+
       - name: Build wheels
         uses: pypa/[email protected]