scanner: add files in include dirs in alphabetical order.

scanner: add files in include dirs in alphabetical order.

This means that if you have a directory structure like this

    /foo
    /foo/02_rules.nft
    /foo/01_rules.nft

where *.nft files in directory /foo are nft scripts, then an include
statement in another nft script like this

    include "/foo/"

guarantees that "01_rules.nft" is loaded before "02_rules.nft".

Signed-off-by: Ismo Puustinen <ismo.puustinen@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>