ATHandler AT debug traces cannot enable in mbed-os 5.12.1

[ccchang12]

Description

Issue request type

[ ] Question
[ ] Enhancement
[X] Bug

For mbed-os-example-cellular example, if set the "cellular.debug-at" to true in JSON file, the building will encounter below error.

Compile [ 6.3%]: AT_ControlPlane_netif.cpp
Compile [ 6.4%]: AT_CellularStack.cpp
Compile [ 6.5%]: equeue_mbed.cpp
Compile [ 6.6%]: ATHandler.cpp
[Error] ATHandler.cpp@1292,0: #20: identifier "malloc" is undefined
[Error] ATHandler.cpp@1322,0: #20: identifier "free" is undefined
[ERROR] ".\mbed-os\features\cellular\framework\AT\ATHandler.cpp", line 1292: Error: #20: identifier "malloc" is undefined
".\mbed-os\features\cellular\framework\AT\ATHandler.cpp", line 1322: Error: #20: identifier "free" is undefined
.\mbed-os\features\cellular\framework\AT\ATHandler.cpp: 0 warnings, 2 errors

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOCUSTRIA-1160

[wajahat-abbas]

This was fixed in this PR.