gui2: Fix cursor setter going out of scope as soon as it's created

Because I'm unbelievably smart, I forgot to name the cursor::setter
variable, so it becomes a temporary that exists for exactly one
statement before the dialog's window is displayed. Addressed this by
giving it a name.

Fixes #4618.

src/gui/dialogs/modal_dialog.cpp

@@ -86,7 +86,7 @@ bool modal_dialog::show(const unsigned auto_close_time)
 	pre_show(*window_);
 
 	{ // Scope the window stack
-		cursor::setter{cursor::NORMAL};
+		cursor::setter cur{cursor::NORMAL};
 		window_stack_handler push_window_stack(window_);
 		retval_ = window_->show(restore_, auto_close_time);
 	}