Mark the GInterface registration delegates CDECL.
Mark the GInterface registration delegates CDECL.

Fixes bug #583901.  Thanks to Markus Henschel for the fix.

* GInterfaceAdapter.cs: add [CDeclCallback] to the delegates

sillsdev/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp,sillsdev/gtk-sharp