Minor fixes

Added dependancy on ftag.h in Makefile. Removed redundant declaration of
struct sqlite3_stmt in ftag.h.
Fix: showing db filename as (null) in -v mode

If one didn't use the -p option, but used verbosity mode, the filename
part of the path in the 'choosing db ...' message had a bug. It would
show as (null) instead of .ftagdb as intended. Fixed by setting the
filename variable, if null, to DB_FILENAME.