Added missing lib to dev UI installation guide
Added missing lib to dev UI installation guide

Horizon doesn't start without this lib.
Error: "ffi.h: No such file or directory"

Change-Id: I02261adbc73b627fd49e152d2de8e9a1c9a3ba95

matips/iosr-2015,redhat-openstack/sahara,tellesnobrega/storm_plugin,mapr/sahara,henaras/sahara,tellesnobrega/storm_plugin,mapr/sahara,zhangjunli177/sahara,henaras/sahara,esikachev/scenario,zhujzhuo/Sahara,citrix-openstack-build/sahara,esikachev/scenario,xme1226/sahara,zhangjunli177/sahara,matips/iosr-2015,henaras/sahara,tellesnobrega/storm_plugin,citrix-openstack-build/sahara,egafford/sahara,tellesnobrega/sahara,keedio/sahara,keedio/sahara,redhat-openstack/sahara,crobby/sahara,ekasitk/sahara,keedio/sahara,xme1226/sahara,bigfootproject/sahara,openstack/sahara,bigfootproject/sahara,egafford/sahara,ekasitk/sahara,matips/iosr-2015,esikachev/sahara-backup,citrix-openstack-build/sahara,tellesnobrega/sahara,bigfootproject/sahara,esikachev/sahara-backup,xme1226/sahara,esikachev/sahara-backup,zhujzhuo/Sahara,crobby/sahara,zhujzhuo/Sahara,esikachev/scenario,zhangjunli177/sahara,redhat-openstack/sahara,openstack/sahara,crobby/sahara,ekasitk/sahara,mapr/sahara