bootmanager: Return value in impossible case

The setting is ranged, so this return statement is unreachable.
But GCC can't tell I guess.

src/yuzu/bootmanager.cpp

@@ -163,6 +163,7 @@ public:
             case Settings::VSyncMode::Mailbox:
                 return 2;
             }
+            return 0;
         }();
 
         format.setSwapInterval(main_surface ? swap_interval : 0);