json: Remove invalid typedef redefinition

json: Remove invalid typedef redefinition

The C standard does not allow for typedef redefinitions, even if they
map to the same underlying type. In order to avoid including the
jsmn.h in json.h and thus exposing jsmn's internals, we have exactly
such a forward-declaring typedef in json.h. If enforcing the GNU99 C
standard, clang may generate a warning about this non-standard
construct.

Fix the issue by using a simple "struct jsmntok" forward declaration
instead of using a typedef.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Tested-by: Chuck Tuffli <chuck@freebsd.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>