input_filter: add missing virtual Source::~Source

Previously the destructors of classes derived from Source were not
called when destructing a Source because Source missed the declaration
of a virtual destructor. This caused at least problems when
reconstructing a Chargen_source with its Chargen_repeater that includes
a One_shot_timeout. In this case, the members of the Alarm behind the
timeout were not reset and the next time beeing used the Alarm appeared
to be still active which wasn't true.

Fixes #2570

repos/os/src/server/input_filter/source.h

@@ -36,6 +36,8 @@ class Input_filter::Source
 
 		Source(Registry<Source> &owner) : _owner_elem(owner, *this) { }
 
+		virtual ~Source() { }
+
 		static bool input_node(Xml_node node)
 		{
 			return node.type() == "input"