package/fail2ban: fix fail2ban-python symlink

Fixes (reproducible):
http://autobuild.buildroot.net/results/50f/50f199bfe06d054cc6770760e73ac0de594a0670/diffoscope-results.txt



Fail2ban installs the fail2ban-python symlink pointing to the host python
intepreter used to run setup.py, which is naturally not valid at runtime and
breaks the reproducible tests as shown in the diffoscope results:

│ -lrwxrwxrwx   0        0        0        0 2020-10-04 10:50:38.000000 ./usr/bin/fail2ban-python -> /home/naourr/work/instance-0/output-1/host/bin/python
│ +lrwxrwxrwx   0        0        0        0 2020-10-04 10:50:38.000000 ./usr/bin/fail2ban-python -> /home/naourr/work/instance-0/output-2/host/bin/python

As a workaround, update the symlink after installation to point to the
correct target python.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>