[mono-config] fix warning (#3820)

[mono-config] fix warning (#3820)

```
mono-config.c:324:14: warning: passing 'const char *' to parameter of type 'void *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                                        g_free (libdir);
                                                ^~~~~~
../../eglib/src/glib.h:125:20: note: passing argument to parameter 'ptr' here
void g_free (void *ptr);
                   ^
1 warning generated.
```