Fix IA32 compilation with common IRQ entry disabled (#1137)

Author: Ryzee119

portable/GCC/IA32_flat/port.c

@@ -666,11 +666,13 @@ static BaseType_t prvCheckValidityOfVectorNumber( uint32_t ulVectorNumber )
         /* In use by FreeRTOS. */
         xReturn = pdFAIL;
     }
+#if ( configUSE_COMMON_INTERRUPT_ENTRY_POINT == 1 )
     else if( xInterruptHandlerTable[ ulVectorNumber ] != NULL )
     {
         /* Already in use by the application. */
         xReturn = pdFAIL;
     }
+#endif  /* configUSE_COMMON_INTERRUPT_ENTRY_POINT */
     else
     {
         xReturn = pdPASS;