libevdev: adjust host-python dependency

libevdev depends on host-python because of some of its build scripts.
These build scripts support python2 and python3.

When python3 is part of the target selection, then we can just use the
interpreter provided by the host-python3 package.

Fixes:
  http://autobuild.buildroot.net/results/dd0/dd04833b11a0ebb0193c861cb375b2112dd339d1/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>