Fix incompatibility with openfortivpn >= 1.11.0

Before openfortipvn 1.11.0 the default configuration was equivalent to:
	--set-dns=1 --pppd-use-peerdns=1
Starting with openfortivpn 1.11.0 the default configuration is equivalent to:
        --set-dns=1 --pppd-use-peerdns=0

NetworkManager-fortisslvpn expects the configuration to be equivalent to:
	--set-dns=0 --pppd-use-peerdns=1
This expectation breaks with openfortivpn 1.11.0 because NetworkManager-fortisslvpn only passes this command line option:
	--no-dns / --set-dns=0
Starting with openfortivpn 1.11.0 it needs to be:
	--no-dns / --set-dns=0 --pppd-use-peerdns=1

This patch adds --pppd-use-peerdns=1 as is already the case in Fedora packages:
https://src.fedoraproject.org/rpms/NetworkManager-fortisslvpn/c/6378487

This is also explained here:
adrienverge/openfortivpn#503
2 jobs for openfortivpn_0.11.0 in 10 minutes and 41 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #637684
fedora28_dist

00:06:46

 
  Test
passed #637685
fedora_from_dist

00:03:54