Fixed warning: "unused-function" "FillSonPaths"

Added a preprocessor if conditon around the definition of "FillSonPaths"
because the only use in this file is contained in such an if condition.
````
 #ifdef __THREEDIM__
static void FillSonPaths (MGIO_RR_RULE *rule)
{

 ...

}
 #endif
````

````
        #ifdef __THREEDIM__
  /* 2D: not even in rm */
  /* son path */
  FillSonPaths(mr);
        #endif
````