Get rid of common syms (#2040)

src/start_menu.c

@@ -75,7 +75,7 @@ enum
 };
 
 // IWRAM common
-bool8 (*gMenuCallback)(void);
+COMMON_DATA bool8 (*gMenuCallback)(void) = NULL;
 
 // EWRAM
 EWRAM_DATA static u8 sSafariBallsWindowId = 0;