DEVELOPERS: add Daniel Lang for package/libsigc

Signed-off-by: Daniel Lang <d.lang@abatec.at>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 189543e4ab6c421001209423cce45c75e3242d75)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

DEVELOPERS

@@ -627,6 +627,7 @@ F:	package/dacapo/
 
 N:	Daniel Lang <d.lang@abatec.at>
 F:	package/dbus-cxx/
+F:	package/libsigc/
 F:	package/paho-mqtt-cpp/
 
 N:	Damien Lanson <damien@kal-host.com>