openzwave: fix compilation of MinOZW

openzwave: fix compilation of MinOZW

LIBDIR was used in cpp/examples/MinOZW/Makefile to add dynamic
libraries. However, as it was unset by default, the cross-compilation
could fail if the host had a library such as libnss3.so in /.
To fix this issue, this patch sets LIBDIR to top_buildir if it is unset

Fixes:
  http://autobuild.buildroot.net/results/68719fdf1320a69310bada6d3c47654dacdb5898

Signed-off-by: Fabrice Fontaine <fabrice.fontaine@orange.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>