package/lttng-tools: fix static build

Some unit tests depend on objects from src/common/.
This patch makes sure to points to the location of the non-PIC object files.
Otherwise, in case of static only build, the PIC object are not built, so
the link will fail.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>