package/python-ply: add host variant

Commit 89e70a7077 (package/bind: fix python build) added a dependency on
host-python-ply to bind, which doesn't exist. Add it.

Fixes:
 - http://autobuild.buildroot.org/results/a68251773f61c3463f4d18aa626c83df70126afc

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Peter: reword / add commit reference]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/python-ply/python-ply.mk

@@ -12,3 +12,4 @@ PYTHON_PLY_LICENSE = BSD-3-Clause
 PYTHON_PLY_LICENSE_FILES = README.md
 
 $(eval $(python-package))
+$(eval $(host-python-package))