Remove warnings with GCC 7

GCC 7 introduced a new warning in switch() statements :
warning: this statement may fall through [-Wimplicit-fallthrough=]

src/buttons_effects.c

@@ -538,7 +538,7 @@ void Button_Grid_menu(void)
         Window_input_content(input_dy_button,str);
 
         Display_cursor();
-
+        break;
       case 7:
         snapgrid = !snapgrid;
         Hide_cursor();