gnome-chess: Drop use of G_MODULE_EXPORT

The usage was incorrect: it resulted in G_MODULE_EXPORT being added to
all function *calls* to these functions, as well as the function
declarations and definitions. Now that G_MODULE_EXPORT is defined as
non-empty on Linux, the build fails.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=780452