build: remove scanner.c and parser_bison.c with `maintainer-clean`

build: remove scanner.c and parser_bison.c with `maintainer-clean`

automake recommends shipping the output of bison and lex in distribution
tar-balls and runs bison and lex during `make dist` (this has the
advantage that end-users don't need to have bison or lex installed to
compile the software).  Accordingly, automake also recommends removing
these files with `make maintainer-clean` and generates rules to do so.
Therefore, remove scanner.c and parser_bison.c from `CLEANFILES`.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>