Replace deprecated gdk_cursor_new

I'm going through old repositories I have laying around and found some old patches for gnome-screenshot (see https://bugzilla.gnome.org/show_bug.cgi?id=759265 and https://github.com/Hjdskes/gnome-screenshot). Most of those have been fixed through other commits, but these hadn't.

This replaces the deprecated gdk_cursor_new function calls with its replacement gdk_cursor_new_for_display.