build/lex-rules.am: Touch the output of the lexer rules

In order not to regenerate lib/cfg-lex.h all the time if cfg-lex.l has
its timestamp updated without any content change, touch the target in
the lexer rules, so that their timestamp gets updated, even if the file
is not regenerated.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>