package/jack2: disable example tools

Disable example tools (enabled by default since bump to version 1.9.20
in commit 26ca7fec and
https://github.com/jackaudio/jack2/commit/508d95a97e0cdfad2e8c7d6cdd478741fdba2c87

)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>