json-c: Add json_object_iterator.h to installed headers

json_object_iterator.h is not installed in staging directory.
This leads to an error during pulseaudio build process:

fatal error: json_object_iterator.h: No such file or directory

Fixes autobuild failures like:
http://autobuild.buildroot.org/results/079f9265cd768f88ad57f6f7b794641c845f3079



Signed-off-by: Gilles Talis <gilles.talis@gmail.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>