graphics/inventor: unbreak with libc++ 3.9

./SoInput.c++:2813:8: error: cannot initialize a variable of type 'char *' with an rvalue of type 'const char *'

char *firstPlus = strchr(n, '+');
      ^           ~~~~~~~~~~~~~~

SoInteractionKit.c++:857:12: error: cannot initialize a variable of type 'char *' with an rvalue of

type 'const char *'
      char *dotPtr   = strchr( partName.getString(), '.' );
            ^          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

SoInteractionKit.c++:858:12: error: cannot initialize a variable of type 'char *' with an rvalue of

type 'const char *'
      char *brackPtr = strchr( partName.getString(), '[' );
            ^          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported by: pkg-fallout