Gio::SignalSocket docs: Add an empty line after @newin
Gio::SignalSocket docs: Add an empty line after @newin

* gio/giomm/socketsource.h: Add an empty line after @newin where it's needed
in order to avoid bad side effects in the documentation. Doxygen assumes that
@newin is followed by a paragraph that describes what is new.

johne53/MB3Glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm