two-step: add unhandled splash mode case to switch

This fixes a compiler warning.

src/plugins/splash/two-step/plugin.c

@@ -205,6 +205,7 @@ view_load_end_animation (view_t *view)
         case PLY_BOOT_SPLASH_MODE_SHUTDOWN:
                 animation_prefix = "shutdown-animation-";
                 break;
+        case PLY_BOOT_SPLASH_MODE_INVALID:
         default:
                 ply_trace ("unexpected splash mode 0x%x\n", plugin->mode);
                 return;