No metrics collected for dialup IPsec VPNs #170

Hi and thank you very much for the great project.

Unfortunately I have noticed that all IPsec VPNs of type "dialup" are skipped entirely during metric collection:

https://github.com/bluecmd/fortigate_exporter/blob/81adda5136cc5f9160c0ec7c9f10ccdcd63713a4/pkg/probe/vpn_ipsec.go#L55-L61

This obviously means that any dialup VPN cannot be monitored, which is the majority in our environment.
I confirmed with a manual curl API call that all our VPNs are returned by the /api/v2/monitor/vpn/ipsec endpoint, so it's just this exporter that then filters them out.

Could this continue statement be removed so that metrics for all IPsec VPNs are collected? I can make a PR for it too if you prefer.