strongswan: swanctl: make overtime local

$overtime has been used since swanctl.init was added in f9d91f1f47.
However, there's no need for it to be global.  Make it local like the
other config variables to avoid polluting the global namespace and make
the code easier to reason about.

Fixes: f9d91f1f47 ("strongswan: migrate to swanctl configs")
Signed-off-by: Kevin Locke <kevin@kevinlocke.name>

net/strongswan/files/swanctl.init

@@ -459,6 +459,7 @@ config_remote() {
 	local local_key
 	local ca_cert
 	local rekeytime
+	local overtime
 	local send_cert
 	local remote_ca_certs
 	local pools