Fix incorrect size compiler warning

[muneebahmed10]

Description:
Compiling core_mqtt_serializer.c gives the following warning:

/Users/ahmemune/Documents/EmbeddedCSDK/libraries/standard/coreMQTT/source/core_mqtt_serializer.c:2218:25: warning: format specifies type 'unsigned long' but the argument has type 'unsigned int' [-Wformat]
                        MQTT_PACKET_PINGREQ_SIZE ) );
                        ^~~~~~~~~~~~~~~~~~~~~~~~
/Users/ahmemune/Documents/EmbeddedCSDK/libraries/standard/coreMQTT/source/core_mqtt_serializer.c:68:53: note: expanded from macro 'MQTT_PACKET_PINGREQ_SIZE'
#define MQTT_PACKET_PINGREQ_SIZE                    ( 2U )

I updated the definition to an unsigned long, matching MQTT_DISCONNECT_PACKET_SIZE