data: Deprecate print storage API

The only API user currently seems to be the examples. fprintd has its
own storage and that will be a good idea in general.

So deprecate the API, we'll need to find a different solution for the
examples eventually.

examples/enroll.c

@@ -142,7 +142,10 @@ int main(void)
 	if (!data)
 		goto out_close;
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
 	r = fp_print_data_save(data, RIGHT_INDEX);
+#pragma GCC diagnostic pop
 	if (r < 0)
 		fprintf(stderr, "Data save failed, code %d\n", r);