Fix build error of analitza on OSX

Insert a cast in free() call on OSX in order to avoid this:
```
Building CXX object analitzaplot/CMakeFiles/AnalitzaPlot.dir/private/utils/marchingcubes.cpp.o
[ 65%] /Users/marko/WC/KDECI-builds/analitza/calgebra/main.cpp:157:3: error: no matching function for call to 'free'
               free(he->line);
               ^~~~
/usr/include/stdlib.h:143:7: note: candidate function not viable: no known conversion from 'const char *' to 'void *' for 1st argument; take the address of the argument with &
void     free(void *);
        ^
Building CXX object analitzaplot/CMakeFiles/AnalitzaPlot.dir/private/abstractplotitem.cpp.o
1 error generated.
make[2]: *** [calgebra/CMakeFiles/calgebra.dir/main.cpp.o] Error 1
make[1]: *** [calgebra/CMakeFiles/calgebra.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
```

REVIEW: 120292