icub.i: Fix compatibility with SWIG 4.2.1

[traversaro]

Without this, compilation with SWIG 4.2.1 fails with:

[100%] Building CXX object bindings/CMakeFiles/_icub_python.dir/CMakeFiles/_icub_python.dir/icubPYTHON_wrap.cxx.o
/home/traversaro/robotology-superbuild/build/src/ICUB/bindings/CMakeFiles/_icub_python.dir/icubPYTHON_wrap.cxx: In function ‘PyObject* _wrap_waveletIntegrand(PyObject*, PyObject*)’:
/home/traversaro/robotology-superbuild/build/src/ICUB/bindings/CMakeFiles/_icub_python.dir/icubPYTHON_wrap.cxx:26621:22: error: ‘waveletIntegrand’ was not declared in this scope; did you mean ‘_wrap_waveletIntegrand’?
26621 |     result = (double)waveletIntegrand(arg1,arg2);
      |                      ^~~~~~~~~~~~~~~~
      |                      _wrap_waveletIntegrand
make[2]: *** [bindings/CMakeFiles/_icub_python.dir/build.make:76: bindings/CMakeFiles/_icub_python.dir/CMakeFiles/_icub_python.dir/icubPYTHON_wrap.cxx.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:4786: bindings/CMakeFiles/_icub_python.dir/all] Error 2
make: *** [Makefile:146: all] Error 2

This is a bit complicated. Basically, waveletIntegrand is a friend function of icub::ctrl::WaveletEncoder , see https://github.com/robotology/icub-main/blob/v2.5.0/src/libraries/ctrlLib/include/iCub/ctrl/functionEncoder.h#L126 . The reason why it is a friend function and not a method is that it needs to be passed to gsl in https://github.com/robotology/icub-main/blob/v2.5.0/src/libraries/ctrlLib/src/functionEncoder.cpp#L119 (I guess). For some reason, SWIG wants to wrap that function, even if it is not declared in the headers (it is directly defined in C++). I guess it is safe to simply ignore the function, that anyhow is not meant to be called by users outside of icub-main.

Related issues:

• apt dependencies from Ubuntu 24.04 support robotology-superbuild#1641 (comment)
• swig 4.2.1 picks up private friend declaration swig/swig#2828

[pattacini]

Thanks!