[minor/bug] add stub functions for HasSeenIP for darwin and windows

These functions need to be implemented, but this fixes current build
errors on these platforms.

Closes #199

modules/netstat/netstat_darwin.go

@@ -173,3 +173,9 @@ func HasListeningPort(port string) (found bool, elements []element, err error) {
 	err = fmt.Errorf("HasListeningPort() is not implemented on %s", runtime.GOOS)
 	return
 }
+
+func HasSeenIP(val string) (found bool, elements []element, err error) {
+	// XXX Currently not implemented for darwin.
+	err = fmt.Errorf("HasSeenIP(): operation is not implemented on darwin")
+	return
+}

modules/netstat/netstat_windows.go

@@ -72,3 +72,9 @@ func HasListeningPort(port string) (found bool, elements []element, err error) {
 	err = fmt.Errorf("HasListeningPort() is not implemented on %s", runtime.GOOS)
 	return
 }
+
+func HasSeenIP(val string) (found bool, elements []element, err error) {
+	// XXX Currently not implemented for windows.
+	err = fmt.Errorf("HasSeenIP(): operation is not implemented on windows")
+	return
+}