Recognize the dbm_open signature without const for a parameter on OpenSUSE

On OpenSUSE, `dbm_open()` expects a `char *` as the file name, instead of a `const char *`.
A warning is emitted during the build:

```
ckpasswd.c:171:25: warning: passing argument 1 of 'dbm_open' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
```