libjson-c: import patch to fix compilation on macos

libjson-c: import patch to fix compilation on macos

Fixes errors in the form of:
  /Users/user/src/openwrt/openwrt/build_dir/hostpkg/json-c-0.16/json_util.c:63:35: error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
  const char *json_util_get_last_err()
                                    ^
                                     void
  1 error generated.
  ninja: build stopped: subcommand failed.

Reported-by: Paul Spooren <mail@aparcar.org>
Suggested-by: Paul Spooren <mail@aparcar.org>
Signed-off-by: Nick Hainke <vincent@systemli.org>