libjson-c: rename the libjson binary package to libjson-c, add a new libjson package with the compatibility library (to deal with the rename)

Backport of r37119

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@37121 3c298f89-4303-0410-b956-a3cf2f4a3e73