package/libcodec2: disable unittest

Quoting
https://svn.code.sf.net/p/freetel/code/codec2-dev/CMakeLists.txt
"# Unittest should be on for dev builds and off for releases."

If unittest is enabled libcodec2 depends on speex and libsamplerate.
To avoid these dependencies we simply disable unittest.

Fixes
http://autobuild.buildroot.net/results/da2/da2b6402c8fabd85a9ee728e8dbf7b78c5aa801e/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>