games/burrtools: unbreak build on 11.0

lib/stl.cpp: In member function 'void stlExporter_c::write(const char*, const voxel_c&, const faceList_c&)':
lib/stl.cpp:69:38: error: invalid conversion from 'const char*' to 'char*' [-fpermissive]

const char * title = basename(fname);
                                   ^

In file included from lib/stl.cpp:44:0:
/usr/include/libgen.h:38:7: error: initializing argument 1 of 'char* basename(char*)' [-fpermissive]
char *basename(char *);

^

PR: 209747
Reported by: antoine (exp-run)