package/libsigrokdecode: fix build with python 3.8

This will fix build with sigrok-cli and python 3.8

Fixes:
 - http://autobuild.buildroot.org/results/4855d42545cbfeeec7529410acc84b61bc8790af



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