package/libs/libjson-c: fix PKG_CPE_ID

package/libs/libjson-c: fix PKG_CPE_ID

cpe:/a:json-c:json-c is the correct CPE ID for libjson-c:
https://nvd.nist.gov/products/cpe/search/results?keyword=cpe:2.3:a:json-c:json-c

Fixes: c61a2395140d92cdd37d3d6ee43a765427e8e318 (add PKG_CPE_ID ids to package and tools)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/15292
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit a4f723e04ed245819fe320f472a4ff2b4eda00fb)
Link: https://github.com/openwrt/openwrt/pull/15881
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>