Resolve build issue for unittest
Resolve build issue for unittest

Error Resolved here is:
SingletonPtr.h:93:13: error: ‘NULL’ was not declared in this scope

kjbracey-arm/mbed,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed