Adjust db_flush_line() definition to avoid clang 15 warning

With clang 15, the following -Werror warnings is produced:

sys/ddb/db_lex.c:94:14: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
db_flush_line()
             ^
              void

This is because db_flush_line() is declared with a (void) argument
list, but defined with an empty argument list. Make the definition match
the declaration.

MFC after: 3 days