[Frontend-GNOME] Use cdecl calling convention for gtkspell p/invokes

MS .NET uses stdcall by default which fails on Windows as GLib based libraries
use cdecl. Mono on Linux on the other hand uses cdecl by default and thus
worked without specifying the calling convention.

Thanks goes to Will Johansson (@johansson) for reporting this issue.
Merge branch 'stable'
[Frontend-GNOME] Use cdecl calling convention for GTK+ p/invokes

MS .NET uses stdcall by default which fails on Windows as GLib based libraries
use cdecl. Mono on Linux on the other hand uses cdecl by default and thus
worked without specifying the calling convention.

Thanks goes to Will Johansson (@johansson) for reporting this issue.