wgengine/monitor: do not ignore changes to pdp_ip*

One current theory (among other things) on battery consumption is that
magicsock is resorting to using the IPv6 over LTE even on WiFi.
One thing that could explain this is that we do not get link change updates
for the LTE modem as we ignore them in this list.
This commit makes us not ignore changes to `pdp_ip` as a test.

Updates #3363

Signed-off-by: Maisem Ali <maisem@tailscale.com>
(cherry picked from commit 80ba161c40)

wgengine/monitor/monitor_darwin.go

@@ -115,7 +115,8 @@ func addrType(addrs []route.Addr, rtaxType int) route.Addr {
 func (m *darwinRouteMon) IsInterestingInterface(iface string) bool {
 	baseName := strings.TrimRight(iface, "0123456789")
 	switch baseName {
-	case "llw", "awdl", "pdp_ip", "ipsec":
+	// TODO(maisem): figure out what this list should actually be.
+	case "llw", "awdl", "ipsec":
 		return false
 	}
 	return true