jansson: Update to version 2.15.0

jansson: Update to version 2.15.0

- Update from version 2.14.1 to 2.15.0
- Update of rootfile
- Changelog
    12.15.0
* New features:
  - Add support for realloc by adding `json_set_alloc_funcs2`, `json_get_alloc_funcs2`
    (@WilhelmWiens in #717)
* Fixes:
  - Optimize serialization (@WilhelmWiens in #658 and #719)
  - Fix docstrings in hashtable.h (@WilhelmWiens in #718)
* Build
  - Use target-based cmake settings (@Andrew-Au in #692)

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>