Fix memory leak

:Release Notes:
Missing virtual destructor for MojServicehandler caused memory leaks.

:Detailed Notes:
Classes inherited from MojRefCounted must have a virtual destructor
in case of smart pointers usage.

:Testing Performed:
tested on local build

:QA Notes:
compile DB8 for x86 and use valgrind

:Issues Addressed:
[ANG-616] DB8 Memory leakage on MojDbServiceHandlerInternal::subscribe()

Open-webOS-DCO-1.0-Signed-off-by: Igor Kuksyuk <igor.kuksiuk@lge.com>

Change-Id: I2f8dac440cd0104c946818f91fd5dae13174d29c
Reviewed-on: https://g2g.palm.com/4467
Reviewed-by: DCO Verification
Reviewed-by: Oleksii Uratkin <oleksii.uratkin@lge.com>
Tested-by: Oleksii Uratkin <oleksii.uratkin@lge.com>