Handle fprintf in handleKnownCallDerivatives

[sbrantq]

Noticed this when trying to run LULESH with __enzyme_autodiff. When compiling the following example with a fprintf, I got an error:Enzyme: No create nofree of empty function (fprintf) fprintf) at context: %3 = call i32 (%struct.__sFILE*, i8*, ...) @fprintf(%struct.__sFILE* noundef %2, i8* noundef getelementptr inbounds ([5 x i8], [5 x i8]* @.str, i64 0, i64 0)) #4 (fprintf) .

#include <cstdio>

extern double __enzyme_autodiff(void*, double);

double fn(double vec) {
  fprintf(stderr, "test");
  return vec * vec;
}

int main() {
    double x = 2.1;
    __enzyme_autodiff((void*)fn, x);
}

Not sure if this is the best fix, though!