level_select_menu.c: Minor compilation warning fix

This fixes the following:
warning: comparison of integer expressions of different signedness: 'int' and 'u32' {aka 'unsigned int'} [-Wsign-compare]

src/menu/level_select_menu.c

@@ -83,7 +83,7 @@ int start_demo(int timer)
     // if the next demo sequence ID is the count limit, reset it back to
     // the first sequence.
 
-    if((++gDemoInputListID_2) == gDemo.animDmaTable->count)
+    if((u32)(++gDemoInputListID_2) == gDemo.animDmaTable->count)
         gDemoInputListID_2 = 0;
 
     gCurrDemoInput = ((struct DemoInput *) gDemo.targetAnim) + 1; // add 1 (+4) to the pointer to skip the demoID.