GDBM_File must cast fatal_func appropriately for the version of gdbm.h

GDBM_File must cast fatal_func appropriately for the version of gdbm.h

The fifth argument to gdbm_open() is an optional callback function for fatal
errors. The prototype for this function has changed between gdbm 1.8.3 and
1.9.0, from void (*)() to void(*)(const char *). This distinction doesn't
matter to a C compiler, but does to a C++ compiler, which we use to test the
core build. So, cast appropriately, depending on the version macros in
gdbm.h