Get rid of common syms (#2040)

src/party_menu.c

@@ -231,7 +231,7 @@ static EWRAM_DATA u16 sUnused = 0;
 EWRAM_DATA u8 gBattlePartyCurrentOrder[PARTY_SIZE / 2] = {0}; // bits 0-3 are the current pos of Slot 1, 4-7 are Slot 2, and so on
 
 // IWRAM common
-void (*gItemUseCB)(u8, TaskFunc);
+COMMON_DATA void (*gItemUseCB)(u8, TaskFunc) = NULL;
 
 static void ResetPartyMenu(void);
 static void CB2_InitPartyMenu(void);