Fix compiler warnings for ext_alloc.c.

After commit 1822c826 started seeing bunch of
warnings like
-------------------------------------------
memaccounting.c:247:1: warning: no previous prototype for function 'MemoryAccounting_DeclareDone' [-Wmissing-prototypes]
MemoryAccounting_DeclareDone()
^
../../../../src/include/utils/memaccounting.h:238:1: note: this declaration is not a prototype; add 'void' to make it a prototype for a zero-parameter function
MemoryAccounting_DeclareDone();
^
                             void
memaccounting.c:263:1: warning: no previous prototype for function 'MemoryAccounting_RequestQuotaIncrease' [-Wmissing-prototypes]
MemoryAccounting_RequestQuotaIncrease()
^
../../../../src/include/utils/memaccounting.h:241:1: note: this declaration is not a prototype; add 'void' to make it a prototype for a zero-parameter function
MemoryAccounting_RequestQuotaIncrease();
^
                                      void
2 warnings generated.
ext_alloc.c:35:1: warning: no previous prototype for function 'Ext_OptimizerAlloc' [-Wmissing-prototypes]
Ext_OptimizerAlloc(size_t size)
^
ext_alloc.c:50:17: warning: unused variable 'account' [-Wunused-variable]
        MemoryAccount *account = MemoryAccounting_ConvertIdToAccount(ActiveMemoryAccountId);
                       ^
ext_alloc.c:48:1: warning: no previous prototype for function 'Ext_OptimizerFree' [-Wmissing-prototypes]
Ext_OptimizerFree(void *ptr)
^
ext_alloc.c:59:1: warning: no previous prototype for function 'GetOptimizerOutstandingMemoryBalance' [-Wmissing-prototypes]
GetOptimizerOutstandingMemoryBalance()
^
-------------------------------------------

src/backend/utils/mmgr/ext_alloc.c

@@ -15,6 +15,7 @@
 #include "utils/gp_alloc.h"
 #include "utils/memaccounting.h"
 #include "utils/memaccounting_private.h"
+#include "utils/ext_alloc.h"
 
 /*
  * This variable is used to track memory that is not freed by Orca during a
@@ -47,7 +48,6 @@ Ext_OptimizerAlloc(size_t size)
 void
 Ext_OptimizerFree(void *ptr)
 {
-	MemoryAccount *account = MemoryAccounting_ConvertIdToAccount(ActiveMemoryAccountId);
 	void *malloc_pointer = UserPtr_GetVmemPtr(ptr);
 	size_t freed_size = VmemPtr_GetUserPtrSize((VmemHeader*) malloc_pointer);
 	MemoryAccounting_Free(ActiveMemoryAccountId, freed_size);

src/include/utils/ext_alloc.h

@@ -24,7 +24,7 @@ extern void*
 Ext_OptimizerAlloc(size_t size);
 
 extern uint64
-GetOptimizerOutstandingMemoryBalance();
+GetOptimizerOutstandingMemoryBalance(void);
 
 
 #ifdef __cplusplus

src/include/utils/memaccounting.h

@@ -235,10 +235,10 @@ extern void
 MemoryAccounting_PrettyPrint(void);
 
 extern uint64
-MemoryAccounting_DeclareDone();
+MemoryAccounting_DeclareDone(void);
 
 extern uint64
-MemoryAccounting_RequestQuotaIncrease();
+MemoryAccounting_RequestQuotaIncrease(void);
 
 extern void
 MemoryAccounting_ExplainAppendCurrentOptimizerAccountInfo(StringInfoData *str);