Add Install build tools and a missed library to the Readme.scons.txt

Add Install build tools and a missed library to the Readme.scons.txt

Add the apt-get command to add the build tools used by the IoTivity
build. This includes valgrind that is only needed in some build
configurations.

libglib2 is required for the  build but seems to be missed in the
readme file as well.

Change-Id: I8fc514a584c7b2134edd55052e4dfe229f2fbf51
Signed-off-by: George Nash <george.nash@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/4813
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Ossama Othman <ossama.othman@intel.com>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>