Recognize the dbm_open signature without const for a parameter on OpenSUSE

[Julien-Elie]

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]