libluna-service2: link explicitly with librt

* transport_shm is using shm_open and when building with eglibc-2.17 it fails
| CMakeFiles/luna-service2.dir/transport_shm.c.o: In function `_LSTransportShmInitOnce':
| src/libluna-service2/transport_shm.c:101: undefined reference to `shm_open'
| src/libluna-service2/transport_shm.c:110: undefined reference to `shm_open'
| collect2: error: ld returned 1 exit status

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@palm.com>
Change-Id: I0b34c7d4f576b162bde661483904672924fb4ed2