src: silence "implicit-fallthrough" warnings

src: silence "implicit-fallthrough" warnings

Gcc with "-Wextra" warns:

    CC       segtree.lo
  segtree.c: In function 'get_set_interval_find':
  segtree.c:129:28: error: this statement may fall through [-Werror=implicit-fallthrough=]
    129 |                         if (expr_basetype(i->key)->type != TYPE_STRING)
        |                            ^
  segtree.c:134:17: note: here
    134 |                 case EXPR_PREFIX:
        |                 ^~~~

    CC       optimize.lo
  optimize.c: In function 'rule_collect_stmts':
  optimize.c:396:28: error: this statement may fall through [-Werror=implicit-fallthrough=]
    396 |                         if (stmt->expr->left->etype == EXPR_CONCAT) {
        |                            ^
  optimize.c:400:17: note: here
    400 |                 case STMT_VERDICT:
        |                 ^~~~

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>