package/json-c: security bump to version 0.15

Fix CVE-2020-12762: json-c through 0.14 has an integer overflow and
out-of-bounds write via a large JSON file, as demonstrated by
printbuf_memappend.

Also update indentation in hash file (two spaces)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>