Fix compiler warning on implicit declaration

:Release Notes:

:Detailed Notes:

:Testing Performed:
Successfully built the code

:QA Notes:

:Issues Addressed:
[GF-7251] pbnjson::JGenerator::serialize leaks memory

Open-webOS-DCO-1.0-Signed-off-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>

Change-Id: I57109d8780096cbe25d2358350576a4073ddcd9d
Reviewed-on: https://g2g.palm.com/2815
Reviewed-by: Build Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-by: seokjun lee <sseokjun.lee@lge.com>
Reviewed-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>