Use MainCallback wherever posible (#2184)

src/battle_main.c

@@ -237,7 +237,7 @@ EWRAM_DATA u16 gBattleMovePower = 0;
 EWRAM_DATA u16 gMoveToLearn = 0;
 EWRAM_DATA u8 gBattleMonForms[MAX_BATTLERS_COUNT] = {0};
 
-COMMON_DATA void (*gPreBattleCallback1)(void) = NULL;
+COMMON_DATA MainCallback gPreBattleCallback1 = NULL;
 COMMON_DATA void (*gBattleMainFunc)(void) = NULL;
 COMMON_DATA struct BattleResults gBattleResults = {0};
 COMMON_DATA u8 gLeveledUpInBattle = 0;