scanner: don't fall back on current directory if include is not found

scanner: don't fall back on current directory if include is not found

This resolves an ambiguity if the same file name is used both under
sysconfdir and the current working directory. You can use dot slash
./ to explicitly refer to files in the current working directory.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1040
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>