package/lttng-tools: fix typo in variable name

It's lttng-tools, not lttng-libust. ;-)

Fixes numerous build failures caused by a late autoreconf:
    http://autobuild.buildroot.org/results/b84/b84a6e39fcca70c56bfe49d54c385dfe6da82422/


    etc...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>