AA: uhttpd: fix build error with the new json-c (#13690)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@36924 3c298f89-4303-0410-b956-a3cf2f4a3e73