vino_background_draw: Make 'initialised' static

The variable 'initialised' is used with g_once_init_enter() to guard the
initialization of the 'background_settings' variable. For this to work,
the 'initialised' variable must be static; otherwise it will be
unpredictable whether 'background_settings' is initialized or not.

https://bugzilla.gnome.org/show_bug.cgi?id=721725