json: Allow to compile with libgpg-error < 1.28

* src/cJSON.c: Use gpgrt fucntion only if available.

We have a hack in gpgme-json to allow building with older libgpg-error
versions. That whole thing will not work but the instead the binary
will print an error at runtime that it needs to be build with a newer
libgcrypt. There was a little bug here for the Debian packages
libgpg-error versions which failed to build cJSON. cJSON is only
needed be the full gpgme-json but nevertheless the Makefile wants to
build it. The fix is straightforward.

• GnuPG-bug-id: T3971
• Signed-off-by: Werner Koch <wk@gnupg.org>