package/libsigrokdecode: fix build with python 3.9

package/libsigrokdecode: fix build with python 3.9

This will fix the following build failure with python 3.9 and
sigrok-cli:

/srv/storage/autobuild/run/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/mips64el-buildroot-linux-uclibc/5.5.0/../../../../mips64el-buildroot-linux-uclibc/bin/ld: /srv/storage/autobuild/run/instance-1/output-1/host/bin/../mips64el-buildroot-linux-uclibc/sysroot/usr/lib/libsigrokdecode.so: undefined reference to `PyList_Insert'

Fixes:
 - http://autobuild.buildroot.org/results/cc6447b926f8223c68d0086428d29a037b18252d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>