fix a const to much:

```
monitord/generate_reports.c:93:22: warning: passing 'const char *' to parameter of type 'void *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                free(mond.reports[s]->r_filter.filename);
                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/stdlib.h:483:25: note: passing argument to parameter '__ptr' here
extern void free (void *__ptr) __THROW;
```