Solved a bunch of -Wall errors on modern

src/battle_bg.c

@@ -692,9 +692,9 @@ static const struct BattleBackground sBattleTerrainTable[] =
     },
 };
 
-static void CB2_UnusedBattleInit(void);
+static void UNUSED CB2_UnusedBattleInit(void);
 
-static void UnusedBattleInit(void)
+static void UNUSED UnusedBattleInit(void)
 {
     u8 spriteId;
 
@@ -704,7 +704,7 @@ static void UnusedBattleInit(void)
     SetMainCallback2(CB2_UnusedBattleInit);
 }
 
-static void CB2_UnusedBattleInit(void)
+static void UNUSED CB2_UnusedBattleInit(void)
 {
     AnimateSprites();
     BuildOamBuffer();