strongswan: fix OpenWrt hotplug script handling

strongswan: fix OpenWrt hotplug script handling

Commit 6cd8fcabe added ipsec hotplug script support by calling "exec
/sbin/hotplug-call ipsec".
Using the exec call breaks the insertion of iptables rules by the _updown.in
script as hotplug-call just replaces the current shell meaning the commands
following exec do not run since the shell is replaced and as a result lead to
connectivity issues.
Fix this by removing the exec command in front of /sbin/hotplug-call.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>