Update fb.c

Fixes error: ‘isc_info_db_SQL_dialect’ undeclared (first use in this function)

replaced in Firebird 3.0 and marked as deprecated from firebird 1.5 left from dark InterBase times 

https://sourceforge.net/p/firebird/mailman/message/27963120/
Merge pull request #28 from mariuz/patch-2

Update fb.c