Nowhere.
After the first successful compilation, v4l2 functions collided with those who were hard part of a kernel (dmesg messages). This made me to compile my own kernel with v4l2 as a module. Then the module compat did not go insert, because of that, that this function already found itself in the kernel (dmesq). So I search where it is and simply delete it.
Maybe it was in reverse order (hack <-> v4l2 as a module). Immemorial, it was late at night (or early morning?) :-D