diagnostics: Fix bootstrap on 32b Darwin hosts.

diagnostics: Fix bootstrap on 32b Darwin hosts.

The change in r16-7507 misses a cast on the value to pp_scalar which
leads to a bootstrap fail on 32b hosts where size_t has a different
declaration from the value.  Fixed by applying the fmt_size_t cast.

gcc/ChangeLog:

	* json.cc (pointer::token::print): Cast the value to pp_scalar
	with (fmt_size_t).

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>