json-c: needs host-autoconf

json-c: needs host-autoconf

The json-c configure script uses the autoheader utility. Use the
host-autoconf provided autoheader, as this tool is not always installed
on the host.

Fixes:
http://autobuild.buildroot.net/results/030/030fddceda4f5bc6379f7bfcc405e92d2e24184a/
http://autobuild.buildroot.net/results/e48/e482f174b8124ace0690a26bb7daf073fe5fcd75/
http://autobuild.buildroot.net/results/482/4829425c38c58d9844aa07108532a93d47191d23/

Cc: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>