Add librt as a link dependency

:Release Notes:
Modified CMake script to add librt to the target dependencies.

:Detailed Notes:
Apparently, luna-sysservice was getting a free linkage to librt via
gthread. Once that dependeny went away, so did the free linkage and
local builds started failing.

:Testing Performed:
Local compile

:QA Notes:

:Issues Addressed:
[GF-52225] luna-sysservice: Coverity reports Medium and Low impact issues

Open-webOS-DCO-1.0-Signed-off-by: Keith Derrick <keith.derrick@lge.com>

Change-Id: I47bc839ae852f94369ae0eb7a8dcc41548e9c0e8
Reviewed-on: https://g2g.palm.com/4002
Reviewed-by: DCO Verification
Reviewed-by: Keith Derrick <keith.derrick@lge.com>
Tested-by: Keith Derrick <keith.derrick@lge.com>