Fix JSON schema 'enum' keyword parsing

:Release Notes:
Added checking of duplicate values for "enum" keyword during JSON Schema
parsing

:Detailed Notes:
To check duplicate values validator compiration functions were
implemented.

:Testing Performed:
Unit tests with memcheck. New unit tests added

:QA Notes:

:Issues Addressed:
[ANG-303] pbnjson: Fix schema keywords validation

Open-webOS-DCO-1.0-Signed-off-by: Mykola Kryvonos <mykola.kryvonos@lge.com>

Change-Id: I2e3457b3c5f30d77c9b83fdaca2c223e714aec51
Reviewed-on: https://g2g.palm.com/4266
Reviewed-by: DCO Verification
Reviewed-by: Mykola Kryvonos <mykola.kryvonos@lge.com>
Tested-by: Mykola Kryvonos <mykola.kryvonos@lge.com>
Reviewed-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-on: https://g2g.palm.com/4619