trace2: fix incorrect function pointer check

Fix trace2_data_json_fl() to check for the presence of pfn_data_json_fl
in its targets, rather than pfn_data_fl, which is not actually called.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trace2.c

@@ -710,7 +710,7 @@ void trace2_data_json_fl(const char *file, int line, const char *category,
 	us_elapsed_region = tr2tls_region_elasped_self(us_now);
 
 	for_each_wanted_builtin (j, tgt_j)
-		if (tgt_j->pfn_data_fl)
+		if (tgt_j->pfn_data_json_fl)
 			tgt_j->pfn_data_json_fl(file, line, us_elapsed_absolute,
 						us_elapsed_region, category,
 						repo, key, value);