package/bind: fix python build

A check for python-ply has been added as this is a dependency of the
dnssec-keymgr script so install host-python-ply to avoid a build failure
if python-ply is not installed on host

Fixes:
 - http://autobuild.buildroot.org/results/96815b1300547c976443bf74b762febdfcc8d3ba



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