tstools: fix build failure in parallel (-j) build

This patch fixes the nightly build failure caused by tstools.  It is
back ported from recent upstream commit
0e8463ea1dc55f566a88bb3df29cf3d5f23cc326.

Once the upstream update the download page with more recent source
tarball, this will no longer be needed.

Signed-off-by: Tzu-Jung Lee <tjlee@ambarella.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>