Use KEY_NONE for buttons without key shortcuts

SDLK_LAST was used, but it is SDL 1.x specific

src/buttons_effects.c

@@ -629,7 +629,7 @@ void Button_Smooth_menu(void)
   Window_display_frame(6,17,130,37);
   for (x=11,y=0; y<4; x+=31,y++)
   {
-    Window_set_normal_button(x,22,27,27,"",0,1,SDLK_LAST);      // 3,4,5,6
+    Window_set_normal_button(x,22,27,27,"",0,1,KEY_NONE);      // 3,4,5,6
     for (j=0; j<3; j++)
       for (i=0; i<3; i++)
         Print_char_in_window(x+2+(i<<3),24+(j<<3),'0'+Smooth_default_matrices[y][i][j],MC_Black,MC_Light);