Use MainCallback wherever posible (#2184)

src/mail.c

@@ -1,7 +1,6 @@
 #include "global.h"
 #include "mail.h"
 #include "constants/items.h"
-#include "main.h"
 #include "overworld.h"
 #include "task.h"
 #include "scanline_effect.h"
@@ -443,7 +442,7 @@ static const struct MailLayout sMailLayouts_Tall[] = {
     },
 };
 
-void ReadMail(struct Mail *mail, void (*exitCallback)(void), bool8 hasText)
+void ReadMail(struct Mail *mail, MainCallback exitCallback, bool8 hasText)
 {
     u16 buffer[2];
     u16 species;