Add leveldb library to build script

Add to build-desktop.sh target to fetch and compile leveldb. In a future
leveldb will be used by db8 as default database backend.

Open-webOS-DCO-1.0-Signed-off-by: Maksym Sditanov <msditanov@luxoft.com>

Change-Id: I12a0d1b5d26b922417b3bce77ed16c8d740deb2c
Reviewed-on: http://g2g.palm.com/1389
Reviewed-by: Build Verification
Reviewed-by: Maksym Sditanov <msditanov@luxoft.com>
Tested-by: Maksym Sditanov <msditanov@luxoft.com>
Reviewed-by: Roger Stringer <roger.stringer@palm.com>